Sceptic User Manual
Contents
1. R1 1100 X gt retract X uw X R2 uw X C lt Antes member X Antes gt 1Ir C retract C lt Antes Stage 4 Negation The above productions only handle dependency rules with positive antecedents We can introduce negation as failure as follows A3 1000 C lt Antes member not X Antes gt 1r C retract C lt Antes R3 uw X Antes gt gt Conses member not X Antes Antes member C Conses gt la C assertz C lt Antes Note the symmetry between A3 and R2 and that between R3 and A2b Stage 5 Multiple supports The above system so far implements logical forward chaining and truth maintenance sym metrically over negated antecedents However productions R2 and A3 logically retract a consequence as soon as one of its support structures is found to be no longer valid i e they assume that any fact only has one support We can enhance the system to allow for multiple supports so that a fact is only retracted when its last support is invalidated R2a uw X C lt Antes member X Antes gt retract C lt Antes uws C lt Antes Ala fw X C lt Antes member not X Antes gt retract C lt Antes uws C lt Antes R4 uws C lt Antes not C lt Other Other Antes gt 1r C The use of extra parentheses to wrap not round several conditions is to enable Prolog to parse the structure correctly as not 1 The complete example This is the comple2. put thinking they are safe Prolog declarations and configurable predicates sc_op 800 xfy lt sc_op 800 xfy sc_op 800 xfy gt gt sc_dynamic f 1 sc_dynamic lt 2 sc_dynamic gt gt 2 objmatch not X A X objmatch X f X 26 The file world pl see later contains details of the agents attributes Load this file before doing anything else Then the agents need to be placed in the world by logically asserting their positions Pursuers will move about at random even if they detect nothing so started is asserted on initialisation to tell them when to start looking Can of course be modified by editing the coordinates of the locations or by adding locations for new agents but remember to add appropriate attributes to the file world pl Once started has been asserted and the locations logically asserted one or more of the productions triggered by verb eoc will always be satisfied when this trigger is generated at the end of the execution cycle The execution of the program can then only be interrupted by C initialise true gt assertz started la location p1 0 0 la location p2 12 12 la location p3 10 5 la location e1 5 5 la location e2 6 6 la location e3 15 16 Actions move_agent N Speed Dir location N 01dX 01dY pos_polar Speed Dir 01dX 01dY NewX NewY gt Ir location N 01dX 01dY
3. reach For example the first ability is described by the rule location A HereX HereY location 0 0x 0y A 0 range A Range pos_polar Dist Dir HereX HereY 0x Oy Dist lt Range gt gt detects A 0 Dist Dir After the belief revision cycle the distinguished trigger eoc will trigger any actions that are now appropriate given the Evaders updated world view For example if an Evader detects any Pursuers it will move away from the nearest Pursuer as fast as it can eoc detects E P Dist Dir evader E pursuer P not detects E P2 Dist2 Dir2 pursuer P2 Dist2 lt Dist speed E Speed NewDir is Dir 180 gt move_agent E Speed NewDir evading _move E P If an action of moving an agent is initiated a brief report is triggered which outputs to the screen the details of the movement Similarly if a Pursuer eats an Evader this fact is also reported Note that some caution is required with the use of eoc triggers If the condition of a rule triggered by eoc is always satisfied there will be a continuous looping of Sceptic execution cycles Thus in this particular example the presence of Pursuers will cause the rule which moves Pursuers randomly 25 eoc started pursuer P not target P E random X Dir is 360 X speed P Speed gt move_agent P Speed Dir random_move P to be executed repeatedly before the locations of the Evaders are logically asserted There will be no break in
4. gt Conses member not L Antes member X L L Antes member C Conses gt la C assertz C lt Antes uws C lt Antes not C lt Other Other Antes gt 1r C Object level database Agents detect all other agents within range location A HereX HereY location 0 0x 0y A 0 pos_polar Dist Dir HereX HereY 0x 0y range A Range Dist lt Range gt gt detects A 0 Dist Dir Pursuers target on the nearest Evader they can see detects P E1 Dist1 Dir1 pursuer P evader E1 not detects P E2 Dist2 Dir2 evader E2 Dist2 lt Dist1 gt gt target P EF1 and could eat any Evader that is within reach detects P E Dist Dir pursuer P evader E reach P Reach Dist lt Reach 29 gt gt could_eat P E The following file is required which contains the Prolog rules which relate the coordinates of one agent to those of another by the distance from and direction of the first to the second A A File polar pl o o ee ee ee ais Quintus specific use_module library math pos_polar Dist Dir Hx Hy Tx Ty var Dist var Dir Xdiff is Tx Hx Ydiff is Ty Hy GDist is Xdiff Xdiff Ydiff Ydiff sqrt GDist Dist Xdiff 0 Radians is 3 14159 2 0 Xdiff 0 D is Ydiff Xdiff atan D Radians des factor Xdiff Ydiff Factor Dir is Factor Radians 3 14159 180 pos_polar Dist Dir Hx Hy Tx Ty var Tx var Ty IDir is integer Dir
5. is the user s responsibility to ensure that suitable conditions are present The eoc facility can be used for example to implement an agenda facility where anything on the agenda is automatically handled at the end of the cycle If items placed on the agenda match the condition agenda Item and the processing of execute agenda Item removes that match the following production will execute the agenda until there is nothing left on it and then terminate eoc agenda Item gt execute agenda Item Syntactically the eoc trigger is optional The above production can also be written agenda Item gt execute agenda Item This is why productions with only a trigger need the dummy condition true Another use of eoc is to construct continuous applications which do not return to the prompt See Appendix B for an example 4 3 Triggers Triggers are not conditions becoming true They are only active as they pass through the execution cycle and are then lost The sequence To Ciy caco On is not to be interpreted as a logical conjunction of conditions with associated expectations of being notified when conditions change Its force is that the conditions C1 Cn are checked and may generate instantiations at the moment when trigger T passes through This is not to say that Sceptic cannot be used to implement logical dependencies and truth maintenance see Appendix A only that the raw Sceptic engine is a simpler forward pr
6. la location N NewX NewY Moves depend on current beliefs An evader moves as fast as it can in a direction opposite to that of the nearest pursuer eoc detects E P Dist Dir evader E pursuer P not detects E P2 Dist2 Dir2 pursuer P2 Dist2 lt Dist speed E Speed NewDir is Dir 180 gt move_agent E Speed NewDir evading_move E P Pursuers chase after evaders they are targeted on eoc target P E detects P E Dist Dir speed P Speed gt move_agent P Speed Dir pursuing_move P E and if they have no targets move about at random just looking This is satisfied merely by the presence of pursuers so the extra 27 condition started is required to guard against continuous cycling of this rule until the user is ready and asserts started eoc started pursuer P not target P E random X Dir is 360xX speed P Speed gt move_agent P Speed Dir random_move P A Pursuer will eat all the Evaders it could eat One could of course have a more complex rule whereby it selected one for example from all the possible eatables eoc could_eat P E gt la is_eaten E lr location E X Y digestion_report E P Y 2523220220922 3232022222232 2232222222382 2S These reports are generated by the appropriate actions evading_move E P location E Xe Ye gt write Evader write E write detects Pursuer write P nl w
7. of triggers This representation is not necessarily a permanent choice It is a compromise which attempts to retain simplicity of structure for explanation and debugging whilst improving efficiency over previous versions by having conditions evaluated as raw Prolog 5 2 Calling Prolog from Sceptic Any Prolog call can be typed to the Sceptic prompt However it will be executed using the Sceptic control which in the current implementation means that all solutions will be generated but without the variable bindings being returned to the user Therefore this method of call is only useful for predicates with side effects a handy example is listing 1 A means of calling normal Prolog i e first solution only and echoing the variable instantiation found is under consideration 5 3 Calling Sceptic from Prolog The top level Sceptic loop can be started by the predicate sc 0 as described above putting the system in Sceptic mode However it may be desirable from within a normal Prolog application to trigger a single Sceptic cycle This is achieved by calling the predicate sc_process 1 For example suppose the input X trigger in mini sc above was the entry point to a more sophis ticated Sceptic application such as a truth maintained logical dependency system A Prolog ap plication could submit a pattern to that application by the call sc_process input Pattern If the Sceptic maintained database in that example were as before fre
8. on the other This is illustrated in considering how a computer model of a rational autonomous agent may be programmed Sceptic would be used to express the control strategies and meta rules for deciding when a decision need be made theorem proved or beliefs about the environment updated However the underlying resolution and unification of Prolog would still be used to prove theorems about the agent s environment on demand So for example in encoding the pursuer evader problem in Sceptic whenever an action oc curs Sceptic productions will be fired which control the revision of the agent s beliefs about the surroundings and the corresponding actions taken as a consequence of the agent s perceptions Pursuers have sensors with a given range and can capture evaders that are within a second given range Evaders also have sensors with a finite range and have set tactics for evading detected pursuers The problem is an interesting vehicle for exploring the behaviour of autonomous and perhaps co operating agents See Appendix B Similarly a truth maintenance system may be succinctly expressed in Sceptic with the produc tions describing the belief revision operations to be carried out when some new information is asserted into or retracted from a knowledge base Prolog as mentioned above will be used to prove any theorems about the world upon which the firing of a production may be conditioned or to obtain any information that is requir
9. the execution cycle during which the user may assert the appropriate locations Thus the extra condition started is included in this rule so that nothing will happen until the user is ready and fires the trigger initialise which results in the assertion of started and the initial locations of the Evaders A complete annotated listing of the demonstration follows Three files are required The file pursuer_evader sc contains the Sceptic productions and object rules for the simulation The file polar p1 is automatically consulted when pursuer_evader sc is loaded and contains the Prolog rules which relate the coordinates of one agent to those of another by the distance from and direction of the first to the second The third file world pl contains the details of the agents attributes and must be loaded before the demonstration is initialised assi tods tie totes doo ett File pursuer_evader sc Requires sc_default_condition X objmatch X A A A eee A A AA Pursuers have attributes speed range reach They can detect any agent that is within range target on the nearest detected evader and eat any evaders that are within reach If they detect no evaders they will move about at random trying to search for one Evaders have attributes speed and range They can detect any agent within range If they detect any pursuers they will turn and run in the opposite direction to the nearest one If they detect no pusuers then they will stay
10. 2 gt gt c _922 d _922 sc gt facts facts none so far first try the rule with negation sc gt la aa 1 sc gt facts check conclusions from absence of b 1 facts aa 1 c 1 d 1 sc gt la b 1 b 1 should cause retractions sc gt facts facts aa 1 b 1 sc gt 1r b 1 retraction of b 1 reinstates conclusions sc gt facts facts aa 1 c 1 d 1 sc gt lr aa 1 sc gt facts facts sc gt now test multiple supports sc gt la p 1 2 sc gt la p 1 3 sc gt facts no conclusions so far facts p 1 2 p 1 3 sc gt la q 2 4 sc gt la q 3 4 sc gt facts facts p 1 2 p 1 3 q 2 4 r 1 4 q 3 4 sc gt supports r 1 4 1 r 1 4 has two supports supports for r 1 4 p 1 2 q 2 4 p 1 3 q 3 4 sc gt 1r q 2 4 sc gt facts facts p 1 2 p 1 3 23 r 1 4 q 3 4 sc gt supports r 1 4 supports for r 1 4 p 1 3 q 3 4 sc gt 1r p 1 3 sc gt facts facts p 1 2 q 3 4 sc gt one support has gone but fact still there last support has gone fact retracted 24 B Pursuer Evader Example This is a simple demonstration of the data driven style of computation that underlies Sceptic Agents have certain attributes and act as their beliefs about the world are updated as a result of changes in the state of the world In this specific example agents belong to one of two classes Pursuers or Evaders Pursuers have attributes speed ran
11. 3 moves randomly to 9 01779 12 0562 Evader e2 detects Pursuer p3 and moves to 21 679 11 7896 Pursuer p3 is targeted on Evader e2 and moves to 14 0147 11 8816 Pursuer pi moves randomly to 15 2194 23 7574 Pursuer p2 moves randomly to 22 2886 36 5701 Evader e2 detects Pursuer p3 29 679 11 7896 and moves Pursuer and moves Pursuer Pursuer Pursuer Pursuer Pursuer to p3 to pl p2 pl p2 ps is targeted on Evader e2 19 014 11 7943 moves moves moves moves moves randomly to randomly to randomly to randomly to randomly to 20 955 15 5658 18 2211 27 4348 19 7364 25 4912 22 9156 18 6052 19 4497 16 7753 whilst pl eats e1 e2 is the only Evader left but no Pursuer can detect it so fall Pursuers move randomly until p3 moves close to e2 and p3 detects and targets on e2 e2 can move a little bit faster than p3 and try as it might p3 can t quite catch it The random movements continue until one of the faster pursuers targets on e2 chases and eats it Then although there are no more Evaders left the Pursuers will continue to move about at random searching in vain The user must type C to interrupt the program once terminal boredom has set in 32 C Command Summary C 1 Top level commands sceptic to start Sceptic from UNIX sc_load 1 load a file vload 1 load a file verbose o
12. 6 7 93537 Evader e3 detects Pursuer p2 and moves to 18 6109 20 7918 Pursuer p1 is targeted on Evader el and moves to 7 07106 7 07105 Pursuer p3 is targeted on Evader e2 Pursuers target on nearest Evader and moves to 5 14853 6 2096 Pursuer p2 is targeted on Evader e3 and moves to 18 0182 19 9863 Evader el detects Pursuer pl and moves to 14 8995 14 8995 Evader e2 detects Pursuer pl 1 The chases continue and moves to 6 11609 9 3246 Evader e3 detects Pursuer p2 and moves to 22 2217 25 5836 Pursuer p1 is targeted on Evader el and moves to 14 1421 14 1421 Pursuer p3 is targeted on Evader e2 and moves to 0 297058 7 4192 Pursuer p2 is targeted on Evader e3 and moves to 24 0363 27 9727 Evader e3 says Eeek Until a Pursuer gets within Pursuer p2 says Yummy reach of an Evader and eats it Evader el detects Pursuer pl and moves to 19 8492 19 8492 Evader e2 detects Pursuer p3 Now only Evaders el and e2 left and moves to 13 6802 11 9292 Pursuer pi is targeted on Evader el and moves to 21 2132 21 2131 Pursuer p3 is targeted on Evader e2 and moves to 5 02464 9 04706 Pursuer p2 moves randomly to 26 2858 37 7163 p2 detects no Evaders so moves randomly 31 Evader el says Eeek Pursuer p1 says Yummy Pursuer p1 moves randomly to 14 5218 13 7817 Pursuer p2 moves randomly to 26 9832 27 7407 Pursuer p
13. Radians is IDir 180 3 14159 cos Radians CDir sin Radians NDir Incx is Dist CDir Incy is Dist NDir Tx is Hx Incx Ty is Hy Incy factor Xdiff Ydiff 180 Xdiff lt 0 Ydiff lt 0 factor Xdiff Ydiff 180 Xdiff lt 0 Ydiff gt O factor _ _ 0 The file world pl contains the details of the agents attributes This may be customised by altering the values of the attributes and by adding or deleting agents pursuer p1 pursuer p2 pursuer p3 speed p1 10 speed p2 10 speed p3 5 range p1 10 range p2 10 range p3 8 reach p1 3 reach p2 3 reach p3 2 evader el evader e2 evader e3 30 speed el 7 speed e2 8 speed e3 6 range e1 9 range e2 9 range e3 10 A sample run The agents attributes and initial locations are as in the listings of the files pursuer_evader sc and world pl given above It will not be possible to reproduce exactly the program run logged here since a random number generator is used for some of the moves As before comments in do not appear on the screen Sceptic Versoin 3 0 sc gt sc_load world pl sc gt sc_load pursuer_evader messages from Prolog whilst oak compiling and consulting files sc gt initialise No more user interaction after initialisation Evader el detects Pursuer pl and moves to 9 94974 9 94974 Evader e2 detects Pursuer p3 Evaders move away from nearest Pursuer and moves to 1 7623
14. Sceptic User Manual September 1989 revised March 1990 Version 3 0 December 1990 Saki Hajnal John Fox Paul Krause Advanced Computation Laboratory Imperial Cancer Research Fund 61 Lincoln s Inn Fields London WC2A 3PX Contents 1 Introduction 2 Syntax 3 Using Sceptic 3 1 Starting the system bliss ac ed et eae A St ee ee ee E a 3 2 Loading a file of productions e 3 3 Escaping to Prolog cossos is ee as 3 4 Typing in extra productions 3 5 Configuring the systeM e 4 Execution AN The CY CIOS gerani ob A A ie ee Ete Se Se 4 2 End of cycle processing ooa ee Ads EROS a ee RS Bae ohh kee a de ath A eae Aad 5 Relation to Prolog Bil Internal form ci Big Poh Ska pclae ee a Se nal A ae a Ba eo 5 2 Calling Prolog from Sceptic ee 5 3 Calling Sceptic from Prolog 0 0 2 0 00000000004 6 Debugging 6 The Sceptic debugger 44 5 66 ges Skee A A Es 6 2 Initial command settings 2 e 6 3 Act phase commands 0 00 gea a E O ee 6 4 Rec phase commands 0 2 0000 2 ee 7 Caveats Gel ROBUStIESS asia eG eS es ee Sh oe RS Gs eed aat T2 CHAR CON CY ER Bae RA eae Re a we a E AR da 7 3 Why not just use Prolog e 7 4 Is Sceptic a specification language 2 o o ee ee 8 Pitfalls for the Unwary A Truth Maintenance Example B Pursuer Evader Example C Command Summary C 1 Top level commands 0 0 00 ee C 2 Configuration
15. accept just a functor name or functor arity combination The productions are displayed in approximately the original syntax Approximately here means that some conditions may be wrapped by sc_one_condition 1 see section 5 1 Internal Form Here is a very small application File mini sc Declare f 2 as dynamic so that it will be recognised as a predicate even before there are any clauses sc_dynamic should simulate this behaviour in the current Prolog sc_dynamic f 2 and 3 3 On the trigger input Patt assert the pattern as a clause for a predicate f Patt Source with Source as u for user but only if that Patt is not already present input X not f X u gt assertz f X u On the trigger showf output an indented list of items entered so far Note that a production needing no conditions other than the trigger has the single dummy condition true showf true gt write items entered so far nl showf f X u gt write gt write X nl an example run remarks in do not appear on the screen sceptic Sceptic Version 3 0 sc gt sc_load mini load application sc gt showf showf is a trigger items entered so far there aren t any yet sc gt input fred input X is a trigger sc gt showf items entered so far fred now both productions for showf fire sc gt input jo sc gt showf items entered so far fred jo sc gt sc g
16. al interest in the Sceptic notation and control mechanism depends on the extent to which it naturally expresses a range of applications If it does then the corresponding direct coding would obscure the common structure underlying those applications If not e g if an application does the meat of its work in complex special conditions coded in Prolog then the extra layer of notation may add nothing 7 4 Is Sceptic a specification language One postulated use of Sceptic was as an executable specification language However writing Sceptic applications still has a large element of programming albeit in a very high level lan guage and as with Prolog Sceptic lacks the type declaration and checking facilities which are the minimal theorem proving capabilities required of a specification language In addition although Sceptic has a very succinct syntax the declarative semantics if any are unclear How ever Sceptic may provide a harness or technique for coding a class of data driven applications in a way which naturally expresses some aspects of those applications The clarity gained may be a helpful step towards specification We have already found cases where a set of Sceptic productions has provided a means of discussing alternative behaviours in a way that larger raw Prolog programs or vaguer English descriptions have not 16 8 Pitfalls for the Unwary The following points have caused problems for several people when learning
17. d and jo already input 10 sc_process input mary yes listing f f fred u f jo u f mary u yes sc_process showf items entered so far fred jo mary yes we are in Prolog but want Sceptic to handle this check what happened showf is also for Sceptic not Prolog 11 6 Debugging 6 1 The Sceptic debugger This section describes the Sceptic oriented debugger The facilities for Prolog debugging depend on the underlying Prolog system The debugger has two phases recognise rec and act By placing spypoints and using mov ing displaying commands and tracking levels you can trace the trigger expansion at various granularities remaining in the act phase At any trigger at which you have paused you can choose to debug the rec phase and trace the evaluation of production conditions also at various granularities Each phase has various commands for moving to a new pause point or displaying information At any pause point typing or h to the debugging prompt will show the available commands Each phase also has an independently set tracking level which controls how much information will be displayed between pause points To start debugging place a spypoint on a trigger The commands are spy Term nospy Term In this version these commands require their argument to be a structure e g spy la _ In the future they may be made to accep
18. displayed between pause points So for example if you continually use command i i e pause at all productions and instantia tions tracking level 1 will have no effect However if you use the new production command p with this level you will see all instantiations passed between then and the next production 15 7 Caveats 7 1 Robustness The system as it stands is very flexible since productions are simply translated into Prolog and conditions and actions may be arbitrary Prolog calls There are few safeguards against clashing predicate names accidental abolition of dynamic declarations etc This can lead to unexpected results However a system with more sophisticated checking mechanisms would probably require more complete and careful declarations from the user This is felt to be premature 7 2 Efficiency The current implementation was designed with some concern for efficiency but other con siderations were given priority There are clearly areas where efficiency could be improved Nonetheless it may turn out that natural expression in Sceptic productions and efficient exe cution are in conflict at least in some cases This is not to say that having such an expression of behaviour would be useless in those cases only that other means might be required to give efficient performance 7 3 Why not just use Prolog Sceptic does nothing in this implementation which cannot be coded directly in Prolog The potenti
19. duction sys tems have to compromise with the requirements of practical programming OPS5 for example has acquired many features and data structures during its development which have obscured its underlying computational model made it a somewhat difficult language to use and its ap plications difficult to maintain Consequently since Props2 was also intended as a serious engineering tool a second aspect of its design was the preservation of a sound declarative style of programming Props2 s emphasis on data driven computation was supported by a truth maintenance system a default mechanism and a number of experimental mechanisms for logical reasoning about control However the specific form of these features was not appropriate for all applications Sceptic has been designed to support a data driven style of computation but without impos ing any constraints on the exact truth maintenance and default mechanisms which may be implemented Sceptic interprets a production syntax and executes productions in a forward or data driven control cycle Sceptic is currently implemented in Prolog and provides full access to the underlying Prolog system As a consequence Sceptic does not in principle do anything that cannot be achieved with Prolog but it does provide a valuable syntactic differentiation between the control and procedural aspects of Prolog on the one hand and the theorem proving Prolog as a logic language and calculation part of Prolog
20. e a spypoint from the current trigger or predicate 33 Display f Full Stack Display the full trigger action stack w Waiting Display the pending triggers predicates only t Current Display the current trigger or predicate This is displayed automati cally before pausing This command is to recall it if other output has intervened Tracking O No tracking Track spied triggers only Track all triggers Track all triggers and predicates WN H C 3 2 Rec phase commands Pause display points New Production Start of a new production for current trigger Instantiation Complete instantiation of current production New Condition Start of a new condition of current production Success Success of current condition Failure Failure of current condition Retry Retry of previous condition Move n Nowait Continue to the end of the rec phase with optional tracking p New Production Stop at the start of the next production or end of rec i Instantiation Stop at the next instantiation or new production or end s Success Stop at the next condition success instantiation production or end c Creep Stop at next point Display There are no separate display commands at present Tracking O No tracking 1 Track productions and instantiations 2 Track productions instantiations and succeeding conditions 3 Track all points 34 D Backwards Compatibility In most respects other than production syntax this version i
21. ed The syntax of Sceptic will be described more fully in the next section By way of introduction Sceptic production rules consist of triggers conditions and actions When a trigger Ty is generated the rules prefixed by Ty Ty C gt A will be fired if the conditions C are satisfied In using Sceptic it is intended that such a rule have an imperative reading If a trigger Ty is signalled and the antecedent C is satisfied at that moment then do A In keeping with this imperative style and the transient nature of the validity of A it is natural to read A as specifying a course of action s such as assert P retract P or fire trigger To Triggers may also be generated by a user As an example we may consider the first production in the truth maintenance system to be described later la X not X gt assert X fw X This may be read To logically assert X the trigger 1a X check first that X is not already known the condition not X If this is the case assert the fact X into the database assert X and then forward chain all consequences of X fw X where fw X is another trigger Sceptic does not at present provide any automated theorem proving capability beyond that available in the underlying Prolog However it is our belief that Sceptic provides a natural and expressive syntax for programming control strategies and belief update procedures In principle any available theorem proving or evaluation strategy ma
22. endix A for an example D 1 4 Renamed configuration predicates The following configuration predicates have been renamed Old Name New Name pprompt sc_prompt default_sceptic_condition sc_default_condition default_sceptic_action sc_default_action default_user_action sc_default_user_action 35 D 2 New functionality not present in previous releases D 2 1 Debugger Version 3 0 is the first version to have a Sceptic oriented debugger D 2 2 Default filename extension Version 3 0 is the first version to have a default filename extension D 2 3 New commands to the Sceptic prompt The following are new commands lp spy nospy oload 36 E Prolog specific Issues and Portability E 1 Prolog portability This version of Sceptic consists of three source files sc_main pl main program sc_mud pl debugger sc_ lt prolog gt pl Prolog specific code e g sc_quintus pl The intention is that all parts which vary between different Prologs should be localised in the sc_ lt prolog gt pl file and that porting to a new Prolog should only involve providing suitable definitions of the predicates in this file This is largely the case although some Prologs may require more extensive alterations e g those which require different comment conventions or other syntax changes The predicates defined in a Prolog specific way are sc_prolog_version PrologAtom PrologAtom describes the Prolog sc_machine_version MachineAtom MachineAtom d
23. escribes the machine architecture sc_init_files List List is a list of initialisation files to be tried in order sc_atom_chars Atom Chars Atom converts to list of characters Chars sc_op Precedence Type Name Used for defining operators since Prologs vary in their argument order for op 3 sc_abolish Functor Arity Remove all trace of the predicate Functor Arity sc_retractall Term Retract all matching clauses but leave the predi cate defined sc_garbage_collect Defined to garbage collect or succeed trivially if not applicable sc_trimcore Defined to trimcore or succeed trivially if not applicable sc_open File Source Open File for reading returning a handle Source sc_read Source Term Read Term from Source sc_close Source Close Source from which you have been reading sc_eof Term Recognise Term as the term returned by sc_read at end of file sc_exists File Succeed if File exists sc_dynamic Functor Arity Simulate a dynamic declaration so that Sceptic will recognise Functor Arity as a predicate even if no clauses exist Other uses are Prolog specific sc_is_predicate Term Succeed iff Term matches a defined predicate whether compiled built in dynamic etc These descriptions are intended simply to indicate the purpose of the predicates For more details necessary to ensure correct behaviour in a port refer to comments in the example files E 2 Application portability The code organisation described abo
24. ge and reach They can detect any agent that is within range target on the nearest detected evader and eat any evaders that are within reach If they detect no evaders they will move about at random trying to search for one Evaders have attributes speed and range They can detect any agent within range If they detect any pursuers they will turn and run in the opposite direction to the nearest one If they detect no pursuers then they will stay put thinking they are safe The agents beliefs are logically maintained using the truth maintenance system that is described in the previous section Each agent has an associated location in Euclidean coordinates and the movement of an agent triggers a revision of the agent s beliefs using the TMS described in Appendix A move_agent N Speed Dir location N 01dX 01dY pos_polar Speed Dir 01dX 01dY NewX NewY gt Ir location N 01dX 01dY la location N NewX NewY The above rule states that to move an agent N with speed Speed in direction Dir from location 01dX 01dY to a new location that is calculated to be NewX NewY then logically retract the old location and logically assert the new location This action will result in a belief revision using the object rules given at the end of the program listing These describe the agents abilities given above viz that they can detect any agent that is within range target on the nearest detected evader and eat any evaders that are within
25. le Loading a second file containing productions with already used trigger names will result in the original productions being overwritten This is analogous to the behaviour of reconsult in Prolog No notion of ownership of triggers by a file is recorded when a file is consulted Thus if all productions of a given trigger are deleted from a previously loaded file and that file then reloaded the corresponding productions will not be removed Mixing default uses As mentioned in Section 6 1 there are only a few built in constraints or checks on the usage of predicate or trigger names the load will only detect clashes between triggers and predicates of the same functor and arity This can lead to difficulties for example if the same predicate name is used in a condition and in an action with a default Sceptic action declared If the predicate has been asserted into the database so that a straight call to it will succeed as a condition the use of the same predicate name as an action may not work as intended The action will be recognised as an existing predicate so the default Sceptic action will not be called For example the default Sceptic action may be declared as revise which removes the current predicate and asserts an updated value 17 sc_default_action R revise R The following Sceptic production is intended as part of an update procedure which calls a last_count predicate increments the value of the associated argument and
26. load 1 load a file old format see Appendix D lt end of file gt escape to Prolog from Sceptic sc 0 restart Sceptic from Prolog pmode 0 enter production entry mode lt end of file gt to return to Sceptic sc_process 1 execute a Sceptic cycle from within Prolog batch 1 run a file as if typed to the prompt 1p 0 1p 1 list productions C 2 Configuration parameters normally in sceptic ini sc_prompt 2 configures top level and production mode prompts de fault values sc gt and scp gt respectively sc_file_extension 2 standard extension for file load default sc sc_default_condition 2 specifies default condition to be used when a condition is not recognised as an existing predicate sc_default_action 2 to be used when an action is not recognised as an existing predicate or trigger sc_default_user_action 2 to be used when a user command is not recognised as an existing predicate or trigger C 3 Debugging commands C 3 1 Act phase commands Move n Nowait Continue to the prompt with optional tracking c Creep Step to next trigger or predicate s Skip Skip over the full expansion of this trigger 1 Leap Leap to next spied trigger sr Skip Recognise Skip over recognise phase for this trigger so you can see the actions generated and still skip afterwards dr Debug Recognise Debug recognise phase for this trigger Spypoints Add Spypoint Place a spypoint on the current trigger or predicate Remove Spypoint Remov
27. n and a prompt here sc gt but configurable Sceptic Version 3 0 sc gt This is an empty system The first step is normally to load a Sceptic application consisting of one or more files of productions or Prolog code 3 2 Loading a file of productions To load a file called test sc type sc gt sc_load test The file extension sc will be used automatically unless the file test exists The choice of extension is configurable A filename with extension may also be given in full with quotes as required by the Prolog parser and must be given if it differs from the default extension sc gt sc_load test 1 An alternative command vload will give verbose feedback on the translation of the input An input file may consist of a mixture of productions in the syntax described above and or dinary Prolog code In particular even if the application is pure Sceptic with no significant Prolog code you are likely to need some minimal predicates and possibly some declarations specific to your version of Prolog although common ones such as dynamic declarations and op erator definitions have been made portable where possible Comments use the normal Prolog conventions or The productions loaded can be inspected using the commands 1p list all productions lp Term list productions matching Term For the moment the command 1p Term requires its argument to be a structure In the future it may be made to
28. n Antes on its own works as a means of producing an instantation of all the antecedents because a list as a single condition is handled as a list of conditions and in each case the default condition is called Both these productions contain the check not X so that no further processing is done if the fact is already present This suggests a re structuring where the check is done once and triggers two actions primitive assertion and forward chaining Ala la X not X gt assertz X fw X A2a fw X Antes gt gt Conses member X Antes Antes member C Conses gt la C Stage 2 Justification maintenance The above productions recursively produce all consequences of a fact which is logically asserted but do not keep track of the dependency relationships between the facts We can simply add to the database a structure which records that a consequence C is supported by an instantiated list of antecedents Antes Here the structure used is C lt Antes where again lt is an arbitrary functor 19 A2b fw X Antes gt gt Conses member X Antes Antes member C Conses gt la C assertz C lt Antes This keeps supports which are accessible in the database but does not yet use them for truth maintenance Stage 3 Logical retract We can use the supports structures added above to define logical retract with two analogous productions uw stands for unwind and is the opposite of fw for forward chain
29. namic gt gt 2 Sceptic productions for logical forward chaining tms system Logical assert references to text 21 Ala A2b A3a la X not X gt a f X fw X fw X Antes gt gt Conses X Antes Antes C Conses gt la C a C lt Antes fw X C lt Antes not X Antes gt r C lt Antes uws C lt Antes Logical retract references to text R1 R2a R3 R4 lr X X gt r f X uw X uw X C lt Antes X Antes gt r C lt Antes uws C lt Antes uw X Antes gt gt Conses not X Antes Antes C Conses gt la C a C lt Antes uws C lt Antes not C lt Other Other Antes gt 1r C Sceptic productions for procedural commands Show current facts facts true gt o facts nl facts f X gt o gt o X nl 7 Show rules rules true gt o rules nl rules Antes gt gt Conses gt o gt o Antes gt gt Conses nl Show supports for X supports X true gt o supports for o X nl supports X X lt Antes gt o 2 o Antes nl Object level database Two dependency rules according to the chosen convention p x D q Y Z gt gt r x D aa X not b X gt gt c X d X A sample run Comments in do not appear on the screen Sceptic Version 3 0 22 sc gt sc_load demo sc gt rules rules p _704 _705 q _705 _710 gt gt r _704 _710 aa _922 not b _92
30. opagation machine driven by events called triggers 5 Relation to Prolog 5 1 Internal form In the current implementation Sceptic productions are translated into clauses for a single Prolog predicate The production T C1 C2 Cn gt Al A2 An becomes approximately the clause sc_satis_prod T A1 A2 An C1 C2 Cn That is there is a satisfied instantiation of a production for T with actions A1 A2 An if conditions C1 C2 Cnaresatisfied The word approximately is used because the con ditions C1 to Cn will only appear unchanged as shown if they are already recognised as Prolog predicates at the time the production is loaded Unrecognised conditions will by wrapped by the predicate sc_one_condition C since they require a runtime check to see if they are predicates or should be handled by the sc_default_condition mechanism This wrapping will be needed either if the condition in question is a predicate not yet defined which will be loaded later or if it is a variable to be instantiated during execution Because of the former order sensitive case efficiency can be increased by loading predicates before productions which use them In the future a multi pass load might be provided to handle this case automatically The top level control mechanism drives the clauses to provide the distinctive features of the system all solutions instantiation and data driven propagations
31. parameters normally in sceptic ini C 3 Debugging commands 0 0000 D Backwards Compatibility D 1 Old functionality now removed 6 2 parra eee ee RA ee ee D 2 New functionality not present in previous releases ii 10 10 10 10 12 12 12 13 14 16 16 16 16 16 17 19 25 33 33 33 33 E Prolog specific Issues and Portability Eel Prolog portability sisao iA A a ee A Se ae E 2 Application portability ooa ee iii 1 Introduction Sceptic draws upon experience using Prolog and Props2 the latter a language developed at ICRF which combined some features of a logic programming language with others more typical of production rule interpreters Prolog is a practical AI programming language but its roots lie in formal logic theorem prov ing and databases One of the main reasons for developing Props2 was the perception that a language for building intelligent automata should also support opportunistic computation computation in response to events occurring unpredictably in time Prolog applications can be programmed to have this characteristic indeed Props2 is written in Prolog but Prolog s de sign emphasises formal logical properties above any other properties considered to be desirable for autonomous systems Production systems which have been widely used for modelling human cognition emphasise event oriented rather than state oriented computations Like Prolog however pro
32. revises the value of the last_count predicate s argument update last_count N N1 is N 1 gt last_count N1 What will actually happen is that when the action is called last_count will be recognised as an existing predicate the default Sceptic action will not be called and instead a straight call last_count N1 will be made This last call will fail as the current value of the argument of last_count is not equal to the value of N1 Old format and eoc only applications In Version 3 0 the trigger eoc is optional so a production without a colon is an eoc production This is syntactically indistinguishable from an ordinary production in the syntax of previous versions In an attempt to help existing users this version detects a load in which no productions have triggers assumes it is an old format application and warns the user accordingly This means that it is not possible to load a new format application which contains only eoc productions 18 A Truth Maintenance Example As remarked above Sceptic productions are not logical dependencies and there is no notification of changes in conditions or truth maintenance The following example uses Sceptic to build up in stages to a logical forward chaining and truth maintenance application similar to part of Props2 5 Stage 1 Logical assert object level forward chaining This example will use the structure Antes gt gt Conses to represent a logical dependency rule where An
33. rite and moves to write Xe write write Ye write nl pursuing_move P E location P X Y gt write Pursuer write P write is targeted on Evader write E nl write and moves to write X write write Y write nl random_move P location P X Y gt write Pursuer write P write moves randomly to write X write write Y write nl digestion_report E P true gt write Evader write E write says Eeek nl write Pursuer write P write says Yummy nl Sceptic productions for logical forward chaining tms system This provides a generic belief maintenance system for all the agents Whenever an agent moves all agents beliefs about the world are updated logical assert la X not X 28 gt assertz f X fw X Antes gt gt Conses member X Antes Antes member C Conses gt la C assertz C lt Antes C lt Antes member not X Antes gt retract C lt Antes uws C lt Antes C lt Antes member not L Antes member X L L gt retract C lt Antes uws C lt Antes logical retract lr X X gt retract f X uw X C lt Antes member X Antes gt retract C lt Antes uws C lt Antes Antes gt gt Conses member not X Antes Antes member C Conses gt la C assertz C lt Antes Antes gt
34. rsts continuing until there are no more triggers to process Note that a All productions matching a trigger are used There is no notion of a cut to indicate that the right production has been found b All instantiations of conditions are found before any actions take place separation of recognise and act phases Therefore for example an assertion action cannot cause a new instantiation to be found on that same burst of expansion c Actions are queued depth first In previous versions this order could be changed to breadth first The effect of expansion order is an open issue which relates to that of potential parallelism Queued actions are interpreted one at a time If they are predicates they are called If they are triggers they are executed as above causing another burst of expansion Unrecognised actions are handled by sc_default_action 4 2 End of cycle processing A distinguished trigger eoc is generated at the end of the cycle If there are productions with the trigger eoc they are processed in the normal way If new triggers are generated and processed then the eoc trigger will be generated again at the end of the new cycle If an eoc trigger generates no new tokens the cycle terminates This may occur either if there are no eoc driven productions or if the productions have conditions which produce no instantiations Note that an eoc production with no other conditions will cause looping It
35. rwards dr Debug Recognise Debug recognise phase for this trigger 6 3 2 Add remove spypoints Once you have stopped at a trigger or predicate you can add a spypoint at it or remove a spypoint from it Add Spypoint Place a spypoint on the current trigger or predicate Remove Spypoint Remove a spypoint from the current trigger or predicate Note that these commands and spy Structure or nospy Structure to the Sceptic prompt will add Sceptic or Prolog spypoints as appropriate However mixed debugging may be con fusing 6 3 3 Display The source of information display in the act phase is a stack structure of pending triggers and actions predicates with information about which triggers generated them The following display commands are available f Full Stack Display the full trigger action stack w Waiting Display the pending triggers predicates only t Current Display the current trigger or predicate This is displayed automati cally before pausing This command is to recall it if other output has intervened The stack is displayed with indentation to reflect the tree structure of the expansion with the root of the tree at the bottom and most indented In the full display triggers which have already run their recognise cycle and generated others and so are no longer waiting are shown in angled brackets Separate instantations in one generation are separated by Consider for example the productions ti c1 X gt
36. s compatible with the previous versions Some changes are documented below D 1 Old functionality now removed D 1 1 Breadth first expansion This version expands actions depth first only This used to be the default behaviour and was used almost exclusively The breadth first option was dropped to simplify development at this stage We may re introduce it later if required The current_expansion_direction parameter is obsolete D 1 2 Production syntax The previous syntax did not distinguish the trigger other than by position That is in old syntax the production Trigger Conditioni ConditionN gt Action1 ActionN was written Trigger Condition1 ConditionN gt Actioni ActionN The eoc trigger had to be included explicitly The normal Sceptic sc_load will expect productions in the new format You can load an old format file using the command oload Filename At the moment there is no save so you cannot load in old format and save in new format automatically but you can use the new command 1p to see the productions in approximately new format and stuff them into a file Approximately here means that some conditions may be wrapped see section 5 1 Internal Form D 1 3 Abbreviations Previous versions contained a small number of built in abbreviations These have been removed but similar functionality can be obtained by defining trivial predicates probably in sceptic ini See App
37. system loads a file called sceptic ini It searches for this file in a sequence of directories current directory user s home directory central Sceptic system directory and loads the first one found Thus the user can provide configuration information for an application current directory for all their applications home directory or rely on the system s default configuration Ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh sceptic ini Default Sceptic configuration file Ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh ESSENTIAL predicates you must have these definitions or replace them sc_prompt top sc gt sc_prompt load scp gt sc_file_extension sc sc_default_condition X sc_error Invalid condition X sc_default_action X sc_error Invalid action X sc_default_user_action X sc_error Invalid user action X OPTIONAL predicates you need these if your application uses them They were built in abbreviations in previous versions of Sceptic o X write X a X assertz X r X retract X X Y member X Y The supplied behaviour for sc_default_condition sc_default_action and sc_default_ user_action is to print an error message on the assumption that all conditions should be defined predicates and all actions should be defined predicates or triggers These configuration parameters can be used in an applica
38. t listing f an example of calling a Prolog predicate f fred u Prolog shows the raw clauses f jo u sc gt Escaping to Prolog The example above is running in Sceptic mode with a top loop prompt sc gt reading triggers and processing them according to Sceptic s execution model This loop can be terminated by lt end of file gt and restarted by the Prolog call sc sc gt D type control D lt end of file gt yes X is 2 3 this is raw Prolog X 5 sc re start the Sceptic loop Sceptic Version 3 0 sc gt To exit all the way to the operating system type lt end of file gt twice 3 4 Typing in extra productions The normal way to load productions is from a file Occasionally it may be convenient to type them directly and a mode is provided for this sc gt pmode go into production mode scp gt test true gt write testing nl type in a valid production input test true gt write testing nl this is verbose feedback sc_satis_prod test write testing nl true showing the internal form scp gt D type control D sc gt test trigger the new production testing sc gt NOTE All productions for a single trigger must be entered at once either from a file or from the keyboard Do not use pmode to add an extra production for an existing trigger the existing definition would be overwritten 3 5 Configuring the system At startup the
39. t just a functor name or functor arity combination The system will stop and prompt at the first occurrence of a spied trigger The prompt is of the following form Phase Level Cmd Name where Phase is rec or act Level is the current tracking level for this phase Cmd is the last command Name is the full name of that command You may input commands see below to the prompt Note that debugging commands are terminated by RETURN and do not require a fullstop Typing RETURN on its own will repeat the last command which is shown in the prompt With the initial commands as described below this may not be the last command typed by the user This aspect is subject to review 6 2 Initial command settings At the start of each act phase debugging cycle i e when you have input something to the prompt the initial command is leap 1 At the start of each rec phase debugging cycle i e when you have chosen the command dr the initial command is new production p 12 6 3 Act phase commands 6 3 1 Move Once you have stopped at a spypoint the following commands are available to move to a new point n Nowait Continue to the prompt with optional tracking c Creep Step to next trigger or predicate s Skip Skip over the full expansion of this trigger l1 Leap Leap to next spied trigger sr Skip Recognise Skip over recognise phase for this trigger so you can see the actions generated and still skip afte
40. t2 X al X t2 X true gt a2 X where a1 X and a2 X are also triggers and the database ci fred c1 jo If we have a spypoint on a2 _ and the user has entered the trigger t1 the structure displayed by the full display is 13 Full Stack a2 fred lt t2 fred gt al fred t2 jo al jo lt t1 gt lt user gt i e a2 fred is the current trigger which was generated from lt t2 fred gt which is no longer pending ai fred belongs to the same instantiation as t2 fred t2 jo and a1 jo are another instantion Both these instantiations were generated from lt t1 gt which is no longer pending which was generated from the user The corresponding waiting display is Waiting a2 fred al fred t2 jo al jo 6 3 4 Track The command to change the tracking level is just to type the level number The available tracking levels are O No tracking 1 Track spied triggers only 2 Track all triggers 3 Track all triggers and predicates The tracking level determines which additional points will be displayed between pause points So for example if you continually leap i e pause at all spied triggers tracking level 1 will have no effect However if you use the nowait command n with this level you will see all spied triggers passed between then and the prompt 6 4 Rec phase commands You enter the rec phase by typing dr at a paused trigger to debug the recognise phase In
41. te example with some command definitions and a pair of object level rules It also demonstrates the use of simple predicates and operator definitions to provide abbrevi ations for example to allow the syntax X Antes for member X Antes Note the various parts of the application a Application specific sceptic ini with configuration predicates 20 b Prolog declarations c Sceptic productions for the main application in this case logical forward chaining tms system d Sceptic productions for procedural commands e Object level database This is a small application and all parts are in sceptic ini plus a single file For larger applications it may be better to separate the parts into separate files STANDARD unchanged from system default file sc_prompt top sc gt sc_prompt load scp gt sc_file_extension sc sc_default_action X sc_error Invalid action X sc_default_user_action X sc_error Invalid user action X APPLICATION SPECIFIC sc_default_condition X objmatch X o X write X a X assertz X r X retract X X Y member X Y objmatch not X X objmatch X f X ne stos css tt ds o soo See ee ae he File demo sc O ee ee Prolog declarations and configurable predicates sc_op 800 xfy lt sc_op 800 xfy sc_op 800 xfy gt gt sc_dynamic f 1 sc_dynamic lt 2 sc_dy
42. tes is a list of antecedents and Conses is a list of consequents The functor gt gt can be used infix with a suitable operator definition It is emphasised that this is just a structure in the database and the choice of functor is arbitrary the more natural choice gt is a Prolog built in in some Prologs We will use database assertions of the form f X to hold facts A suitable definition of sc_default_condition will cause conditions to default to matching on this structure We wish to define a trigger la X which will perform logical assertion of a proposition X i e add X to the database together with all its consequences as implied by the dependency rules present Al la X not X gt assertz f X A2 la X not X Antes gt gt Conses member X Antes Antes member C Conses gt la C Production A1 primitively asserts the fact X structure f X if it is not already present in the database The check for absence suppression of duplicates is a feature of Props2 5 and corresponds to the notion that once a fact is known it makes no sense to re assert its truth Production A2 does the forward chaining on object level rules The use of member 2 corre sponds to the representation of antecedents and consequents as lists Note that patterns unify as in Prolog so that if member X Antes succeeds X matches one of the Antes any embedded variables in that antecedent will be unified with the values in X The conditio
43. this phase there are no spypoints as such There are various distinguished points in the cycle which are available as pause points These points are New Production Start of a new production for current trigger Instantiation Complete instantiation of current production New Condition Start of a new condition of current production Success Success of current condition Failure Failure of current condition Retry Retry of previous condition 14 On starting to debug the rec phase the first pause is at the start of the first production The different commands and levels pause or display different subsets of these points 6 4 1 Move Once you have stopped at a point the following commands are available to move to a new point n Nowait Continue to the end of the rec phase with optional tracking p New Production Stop at the start of the next production or end of rec phase i Instantiation Stop at the next instantiation or new production or end s Success Stop at the next condition success instantiation production or end c Creep Stop at next point 6 4 2 Display There are no separate display commands at present 6 4 3 Track The command to change the tracking level is just to type the level number The available tracking levels are O No tracking 1 Track productions and instantiations 2 Track productions instantiations and succeeding conditions 3 Track all points The tracking level determines which additional points will be
44. tion specific way to define non trivial default behaviours For example sc_default_condition can be used to specify that all conditions which are not recognised existing predicates should be interpreted as matches on some object level database by replacing the line in sceptic ini by sc_default_condition X objmatch X and providing the definition of objmatch 1 For example for a system which expects an object level database of facts represented by clauses of the form f Patt Source an appropriate definition may be objmatch not X A X _ objmatch X f X _ Similarly the definition default_user_action X user X means that any user input which is not recognised as a defined predicate or trigger will be handled by user 1 The definition of user 1 is up to the application builder e g if we add to the example mini sc above the production user X true gt input X the user no longer has to type input Patt and the session becomes sc gt showf items entered so far sc gt fred will call user fred sc gt showf items entered so far fred sc gt 4 Execution 4 1 The cycle The execution of Sceptic productions is as follows For any trigger each production matching that trigger is taken in turn All instantiations of its conditions are found and the corresponding actions queued for execution This constitutes a single burst of expansion A Sceptic cycle consists of a series of such bu
45. to use Sceptic They should be borne in mind when starting to write programs in Sceptic Non instantiation of variables in actions Variables which are uninstantiated when an action is called will not be bound after the call For example the production write_sum A B true gt C is A B write A B write C nl will give the following response sc gt write_sum 1 2 A B _1700 whereas the production write_sum A B C is A B gt write A B write C nl will respond as intended sc gt write_sum 1 2 A B 3 This is a result of the way in which Sceptic is implemented and is a possibly controversial feature Currently predicate actions are called one at a time for all solutions so that although the addition takes place and succeeds it fails on retry and the variable C is once again uninstantiated by the time the next action is reached The rationale for this behaviour is that instantiation of variables should remain an aspect of checking the satisfaction of conditions Read the above rule as to write out the sum of two numbers if C is their sum then output C Dynamic Predicates Predicates which are not in existence when an application is loaded but which will be asserted during the Sceptic session must be declared as dynamic since Sceptic behaviour relies on distinguishing predicates and non predicates Multiple use of Triggers All productions with the same trigger name must be loaded from a single fi
46. ve is intended to facilitate portability of Sceptic itself between versions of Prolog However this is insufficient to ensure portability of Sceptic appli cations 37 Sceptic allows full access to the underlying Prolog and therefore does not prevent the appli cation writer from using any aspect of that Prolog version including possibly unwittingly non portable aspects If such aspects are identified and if application portability is an issue an approach analogous to the above can be taken using file s of library predicates with versions coded for different Prologs 38
47. y be called within the execution of a Sceptic production The value of Sceptic is in programming the event oriented activities of an autonomous system This manual describes version 3 0 of Sceptic For changes from previous versions refer to Appendix D 2 Syntax The Sceptic notation consists of productions of the following form T Ci C2 Cn gt Al A2 An where T is a trigger C1 Cn are conditions Ai An are actions gt is a reserved symbol and fullstop is a terminator Triggers conditions and actions may contain embedded variables and there is an implicit universal instantiation i e a production fires for all instantiations of its conditions A condition may in the current implementation be any Prolog predicate with a configurable default condition This may be used for example to provide an object level match i e any condition which is not a defined predicate is taken to be a match on some database where the precise definitions of the match function and database form are configurable An action may also be any Prolog predicate a further trigger or a default action which is again configurable For example the following production will write to the screen all clauses of the form f X Y showf X Y gt write f X Y nl 3 Using Sceptic 3 1 Starting the system To start the system type sceptic to the unix prompt here The system starts with a message stating the current versio
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