Regression in Modal Logic Robert Demolombe* - Andreas Herzig** - Ivan Varzinczak** * ONERA Toulouse 2 Avenue Edouard Belin, B.P. 4025 31055 Toulouse Cedex France Robert.Demolombe@cert.fr ** Institut de Recherche en Informatique de Toulouse 118, route de Narbonne 31062 Toulouse Cedex 4 France {herzig,ivan}@irit.fr ABSTRACT. In this work we propose an encoding of Reiter's Situation Calculus solution to the frame problem into the framework of a simple multimodal logic of actions. In particular we present the modal counterpart of the regression technique. This gives us a theorem proving method for a relevant fragment of our modal logic. KEYWORDS: reasoning about actions, regression, modal logic, dependence. 1. Introduction In the reasoning about actions field most approaches use the Situation Calculus formalism [MCC 69]. Among those, Reiter's [REI 91] has turned out to be most fruitful. His basic formalism is restricted to deterministic actions without ramifications. In order to solve the frame problem he makes use of so-called successor state axioms (SSAs). The latter enable regression [REI 91], which has interesting computational properties. The Situation Calculus is a dialect of predicate logic, having situations and actions as objects, and where actions are viewed as mappings on the set of situations. At first glance this is very close to possible worlds semantics for Deterministic PDL [HAR 84]. But the precise relation between Reiter's approach and dynamic logic is not as obvious as that. One of the reasons why his formalism cannot be translated straightforwardly into modal logics of action such as PDL is that the Situation Calculus allows quantifying over actions. Worse, such quantifications are central to Reiter's approach. Journal of Applied Non-Classical Logics.Volume 13 – n◦ 2/2003, pages 165 to 185 166 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 In [DEM 03] there has been presented a technique to translate Reiter's approach into dynamic logic. In this paper we present a different approach. We solve the problem using an extension of dynamic logic that has been introduced in [CAS 99]. There, dynamic logic is combined with a causal notion based on a dependence relation, resulting in a family of logics LAP;. LAP; is a simple yet powerful account to the frame and ramification problems, with the advantage of having a decision procedure in terms of tableau systems (while the Situation Calculus contains second-order axioms and is a priori not even semi-decidable). We propose an encoding of Reiter's approach into the formalism of LAP;. Having such a result provides some degree of optimization in doing inference tasks for some classes of problems in the area. This work is organized in the following way: in Section 2 we present a slightly modified version of PDL, which will serve as the basis for developing the central ideas of this paper. Section 3 is devoted to introduce the basic hypotheses concerning the knowledge we have about actions. In Section 4 we present Reiter's solution to the frame problem in the logical basis of Section 2 and in Section 5 we summarize Reiter's regression technique. We then revisit De Giacomo and Lenzerini's account for encoding domain descriptions into a variant of dynamic logic that avoids quantification over actions (Section 6). In Section 7 we present our modal logic of actions LAP;. In Section 8 we show how we can do regression in LAP;. Finally we sketch possible extensions to this work (Section 9) and then give some concluding remarks. A preliminary version of this paper has been presented at the workshop 'Methods for Modalities' (M4M-3). Thanks are due to Jérôme Lang for comments and discussions. 2. Deterministic PDL with quantification and equality In this section we introduce a slightly extended version of deterministic PDL containing quantification over actions and the equality predicate. We use P1, P2, . . . for propositional constants, and P,Q, . . . as metavariables for propositional constants. A1, A2, . . . denote action constants and a, b, . . . action variables. We will use A,B, . . . as metavariables ranging over action constants and variables. PRP is the set of all propositional constants, and ACT is the set of all action constants. Examples of propositional constants are Loaded ("the gun is loaded") and Alive ("the agent is alive"). Examples of action constants are shoot ("shooting the agent") and strangle ("strangling the agent"). L1, L2, . . . denote literals. If L = ¬P then we shall identify ¬L with P . φ, ψ, . . . denote formulas that are constructed in the usual way from PRP using the classical propositional operators. We shall also call them classical formulas. Hence they do not contain modal operators, quantifiers or the equality predicate that is to be introduced below. We will use modal operators [A] (resp. [a]) for each action constant A ∈ ACT (resp. action variable a). Φ,Ψ will denote complex formulas possibly involving modal operators, quantification, and equality between actions. [A]Φ is read "after executing Regression in Modal Logic 167 A, Φ". We also use the dual 〈A〉 of [A]. The formula 〈A〉> can be read as "A is executable". The nonstandard feature of our logic is that we allow for quantification over actions, and for equality between actions. Hence, in this version of dynamic logic we allow for formulas of the form ∀aΦ, with Φ a complex formula as defined above. In the Yale shooting scenario (YSS) [HAN 86], one can e.g. write ∀a(Alive ∧ ¬[a]Alive→ (a = shoot ∧ HasGun ∧ Loaded)). This is an explanation closure axiom [SCH 90] expressing that the only way to make Alive false is by the shooting action. A model is a triple M = 〈W,R, I〉 where W is a set of Kripke possible worlds, R is a set of binary relations on W , and I is an interpretation function mapping propositional constants to subsets of W , and action constants and variables to elements of R. We will sometimes write w′ ∈ (I(A))(w) instead of wI(A)w′ , and similarly for variables a. We say that the interpretation I agrees with I ′ except possibly on a if and only if – I(P ) = I ′(P ) for every propositional constant P ; – I(A) = I ′(A) for every action constant A; – I(b) = I ′(b) for every action variable b different from a. For a given model M = 〈W,R, I〉, w |=M ∀aΦ if for every I ′ such that I agrees with I ′ except possibly on a, w |=〈W,R,I′〉 Φ. w |=M [A]Φ if for every w′ ∈ (I(A))(w), w′ |=M Φ. w |=M [a]Φ if for every w′ ∈ (I(a))(w), w′ |=M Φ. We say that a formula Ψ is a consequence of the set of global axioms {Φ1, . . . ,Φn} in the class of modelsM (noted {Φ1, . . . ,Φn} |=M Ψ) if and only if for all M ∈M, if |=M Φi for every Φi, then |=M Ψ. We will use K to denote the class of all possible models. DK = {〈W,R, I〉 ∈ K : R is a partial function} is the class of models where actions are deterministic, i.e., (I(A))(w) is either a singleton or empty. Thus, for all action constants A and all formulas Φ |=DK 〈A〉Φ→ [A]Φ (1) If all actions are deterministic, then every formula without quantification can be brought into a normal form where there are neither conjunctions nor disjunctions in the scope of modal operators. Apart from classical equivalences, this uses the following ones from the left to the right: |=DK [A](Φ ∧Ψ)↔ ([A]Φ ∧ [A]Ψ) (2) |=DK [A](Φ ∨Ψ)↔ ([A]Φ ∨ [A]Ψ) (3) 168 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 3. Describing actions Reiter (and more generally the reasoning about actions community) focuses on deductions from a theory describing a given set of actions in terms of preconditions and effects. In dynamic logic such an action theory corresponds to a set of global axioms in Fitting's sense [FIT 83]. We have for example, {[load]Loaded, Loaded→ [shoot]¬Alive} |=K [load][shoot]¬Alive In the Situation Calculus, the same result is obtained by quantifying over situations. For our running example we have ∀sLoaded(do(load, s)) and ∀s(Loaded(s)→ ¬Alive(do(shoot, s))), where s is a variable of sort situation, do is a function symbol, load, shoot are constants of sort action, and Loaded, Alive predicate symbols. In describing an action theory it is more or less explicitly supposed that the following pieces of information are given. About these items some assumptions of complete information are made. 3.1. Action preconditions For each action constantA there is a classical formula Poss(A) describing the action precondition ofA, i.e. the condition under whichA can be executed. For example Poss(shoot) = HasGun, and Poss(strangle) = >. It is supposed that the action preconditions are complete: A is executable if and only if Poss(A) is true. In terms of dynamic logic, completeness of action preconditions means that for every A ∈ ACT we have a global axiom Poss(A)↔ ¬[A]⊥. 3.2. Set of possible causes For each propositional constant P there are two finite sets of action constants Cause +(P ) and Cause−(P ) describing the positive and negative causes of P . (Note that ACT may be infinite.) Cause+(P ) contains the actions in ACT which in some circumstances might cause P to become true, while Cause−(P ) contains those actions that may cause P false. For example Cause+(Alive) = ∅ (no action makes an agent alive), Cause−(Alive) = {shoot, strangle}, and Cause−(Loaded) = {shoot}.1 It is also supposed that Cause+(P ) and Cause−(P ) are small, in the sense that Cause +(P ) and Cause−(P ) are much smaller than ACT . Moreover, we suppose that these two sets are complete: whenever A 6∈ Cause +(P ) then the execution of A can never make P true. In terms of dynamic 1. In Reiter's presentation these functions retrieved from his functions γ+ and γ−. Regression in Modal Logic 169 logic, causal completeness means that we have a global axiom ¬P → [A]¬P in that case. Similarly, for every B such that B 6∈ Cause−(P ) we have a global axiom P → [B]P . Axioms of that form are called frame axioms. In our example, as strangle 6∈ Cause−(Loaded), we have Loaded→ [strangle]Loaded. The next piece of information specifies the causal relation in more detail. 3.3. Effect preconditions For all propositional constant P ∈ PRP and every action constant A ∈ Cause +(P ) there is a classical formula Cond+(A,P ) describing the positive effect precondition of action A. Similarly, for every A ∈ Cause−(P ) there is a Cond−(A,P ) describing its negative effect precondition. For example Cond −(strangle,Alive) = >, and Cond−(shoot,Alive) = Loaded.2 It is supposed that the effect preconditions are complete: in situations where the formula Cond+(A,P ) does not hold the execution ofA can never make P true. Symmetrically, when Cond−(A,P ) does not hold then the execution ofA can never make P false. In terms of dynamic logic, to every effect precondition Cond+(A,P ) one can associate a global axiom Cond+(A,P ) → [A]P , and to every effect precondition Cond −(A,P ) one can associate a global axiom Cond−(A,P ) → [A]¬P . As an example, consider the formula Loaded→ [shoot]¬Alive. Completeness of effect preconditions means that we moreover have a global axiom (¬Cond+(A,P ) ∧ ¬P ) → [A]¬P for every A ∈ Cause+(P ). Symmetrically, for everyB such thatB ∈ Cause−(P ) we have a global axiom (¬Cond−(B,P )∧P )→ [B]P . For example we have (¬Loaded ∧ Alive)→ [shoot]Alive. 3.4. Comments The last two completeness assumptions of Sections 3.2 and 3.3 express in modal logic what Reiter calls "explanation closure" and "Clark completion". Most importantly, the three pieces of information together with the completeness assumptions make that the possible world resulting from the execution of action A in a possible world w is completely determined: for every model M and world w of M , if w 6|=M Poss(A) then (I(A))(w) = ∅. Else the truth value of every P in every w′ accessible from w via I(A) is as follows. Suppose w.l.o.g. that w |=M P . Then: – if A 6∈ Cause−(P ) then w′ |=M P ; – if A ∈ Cause−(P ) and w 6|=M Cond −(A,P ) then w′ |=M P ; – if A ∈ Cause−(P ) and w |=M Cond −(A,P ) then w′ 6|=M P . 2. These functions correspond to Reiter's γ+ and γ−. 170 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 As all truth values are thus determined, it follows that the set of worlds accessible via I(A) is either empty, or it can be considered to be a singleton. This fits with the assumption that all actions are deterministic. As we have noted, the action preconditions and effect preconditions appear explicitly in Reiter's formalization, while the sets of possible causes Cause+(P ) and Cause −(P ) only appear implicitly there. Note that in Reiter's Situation Calculus it is supposed that actions always lead to some state: even in states where the agent has no gun in his hands, the state resulting from the execution of shoot exists. The technical reason is that just as every function in predicate logic, his successor function do is total. This means that the logic of each action operator [A] should be KD. We have nevertheless decided to follow the dynamic logic tradition and suppose that the set of worlds accessible via some action A might be empty. Therefore the logic of each [A] is just K. In fact, inexecutability of the action shoot is expressed in Situation Calculus by stating Poss(shoot)↔ HasGun, where Poss(shoot) is a particular propositional constant. In our formulation, Poss is a function associating a classical formula to every action A. Poss(A) can be seen as an abbreviation, such as Poss(shoot) = HasGun. Given a domain description in Reiter's style, we obtain a description in our style if we (1) define our Poss-function from Reiter's preconditions Poss(A) ↔ φ, and (2) replace Reiter's constants Poss(A) by our 〈A〉>. The other way round, our version can be translated to Reiter's by (1) defining his preconditions Poss(A) ↔ φ from our Poss-function, and (2) recursively replacing [A]φ by Poss(A) → [A]φ. Observe that the latter is nothing but the well-known translation from modal logic K to KD [OHL 91, OHL 93]. All this sounds as if action theories could be described in deterministic PDL in a satisfactory manner, but we have not solved the frame problem yet: as by hypothesis Cause +(P ) and Cause−(P ) are small, it follows that the size of the set of frame axioms that we have to state is close to card(PRP) × card(ACT ). This is usually considered to be too big, and a central element in the research program of the reasoning about actions community was to design mechanisms allowing to infer such frame axioms without stating them explicitly. There was a 20-years-long debate about semantics and theorem proving methods allowing such inferences. Reiter's proposal seems to have closed the debate at least in what concerns deterministic actions without side-effects (also called ramifications). This is going to be presented in the sequel. 4. Reiter's solution to the frame problem Based on a particular class of models, Reiter proposes to incorporate the basic ingredients of action theories that we have presented in the preceding section into Regression in Modal Logic 171 successor state axioms which given a state and an action completely determine the next state. 4.1. Reiter models Reiter requires that names are unique and that models are trees. Thus, given a model M = 〈W,R, I〉, we say that M is a Reiter model if and only if 〈W, ⋃ r∈R r〉 is a tree, and if I(Ai) = I(Aj), then i = j. RT R will denote the class of all Reiter models. 4.2. Successor state axioms Suppose that all the Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ) and Cond −(A,P ) are given, and that the completeness assumptions are made. We then can associate with that an action theory R from which the relevant frame axioms will follow. In dynamic logic R is made of the following axioms: – for every A ∈ ACT , there is an executability axiom Poss(A)↔ ¬[A]⊥; – for every P ∈ PRP , if Cause+(P ) = {A1, . . . , An} and Cause −(P ) = {B1, . . . , Bm} then there is a successor state axiom ∀a([a]P ↔ (¬Poss(a) ∨ (a = A1 ∧ Cond +(A1, P )) ∨ . . . ∨ (a = An ∧ Cond +(An, P )) ∨ (P ∧ ¬(a = B1 ∧ Cond −(B1, P )) ∧ . . . ∧ ¬(a = Bm ∧ Cond −(Bm, P ))))) Note that the successor state axiom is well defined because we have supposed that Cause +(A) and Cause−(A) are finite. For the cases where n = 0 or m = 0, conjunction of the elements of an empty set is identified with >, and disjunction with ⊥. The latter can be illustrated with our running example, where Cause+(Alive) = ∅. The successor state axiom for Alive is: ∀a([a]Alive↔ (¬Poss(a) ∨ ⊥ ∨ (Alive ∧ ¬(a = shoot ∧ Loaded) ∧ ¬(a = strangle ∧ >)))) We abbreviate Reg(a, P ) the right hand side of the equivalence. The successor state axiom for P therefore has the form ∀a([a]P ↔ Reg(a, P )). 172 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 Successor state axioms can be equivalently stated for negative literals as: ∀a([a]¬P ↔ (¬Poss(a) ∨ (a = B1 ∧ Cond −(B1, P )) ∨ . . . ∨ (a = Bm ∧ Cond −(Bm, P )) ∨ (¬P ∧ ¬(a = A1 ∧ Cond +(A1, P )) ∧ . . . ∧ ¬(a = An ∧ Cond +(An, P ))))) We abbreviate Reg(a,¬P ) the right hand side of this equivalence. For example the successor state axiom for ¬Alive is: ∀a([a]¬Alive↔ (¬Poss(a) ∨ (a = shoot ∧ Loaded) ∨ (a = strangle ∧ >) ∨ (¬Alive ∧ ¬⊥))) 4.3. Comments Reiter's original axiom [REI 91] is slightly different from ours: ∀a(Poss(a)→ ([a]P ↔ ((a = A1 ∧ Cond +(A1, P )) ∨ . . . ∨ (a = An ∧ Cond +(An, P )) ∨ (P ∧ ¬(a = B1 ∧ Cond −(B1, P )) ∧ . . . ∧ ¬(a = Bm ∧ Cond −(Bm, P ))))) Our version can be proved to be equivalent to his. In his book [REI 01] Reiter excluded the precondition Poss(a) from the right hand side Reg(a, P ) of the SSA, and just writes ∀a([a]P ↔ ((a = A1 ∧ Cond +(A1, P )) ∨ . . . ∨ (a = An ∧ Cond +(An, P )) ∨ (P ∧ ¬(a = B1 ∧ Cond −(B1, P )) ∧ . . . ∧ ¬(a = Bm ∧ Cond −(Bm, P )))) Therefore we would have e.g. [shoot]¬Alive↔ (Loaded∨(¬Alive∧¬⊥)), from which it follows by classical principles that ¬HasGun ∧ Alive ∧ [shoot]¬Alive → Loaded. This means that such SSAs do not take into account inexecutability: this issue must be managed "by hand" by introducing Poss(shoot) atoms in the right places when proving consequences of SSAs in their recent version. Finally, we note that Reiter's presentation also contains precondition axioms of the form Poss(A) ↔ φ. This is not needed here because we view Poss(A) as a function returning a classical formula φ, which is directly integrated into our successor state axiom (cf. also our comments in section 3.4). 5. Reiter's regression Successor state axioms are crucial when it comes to the reasoning aspect of the frame problem, to which we turn now. Regression in Modal Logic 173 Given a Reiter's style action theory R, what can be deduced from it? Suppose Φ is a complex formula without quantification, action variables, and equality, such as HasGun → [load][shoot]¬Alive. In order to decide whether R |=RT R Φ, Reiter proposes to rewrite Φ using the successor state axioms from the left to the right. This is what he calls regression, and it consists in syntactical substitutions whose iteration reduces a given formula with action symbols into another one with just propositional constants. At each regression step we have to put formulas in normal form such that there are neither conjunctions nor disjunctions in the scope of modal operators (using the hypothesis that all actions are deterministic). Hence the innermost modal operators have just literals in their scope. For the above example, Φ gets ¬HasGun ∨ [load][shoot]¬Alive. ALGORITHM 1 (REITER'S REGRESSION). - input: – a formula without variables Φ. – Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ) and Cond−(A,P ). output: a classical formula REG(Φ). begin while Φ is not classical put Φ in normal form choose a subformula [A]L, where L is either P or ¬P , for P ∈ PRP if L = P then replace [A]P by Reg(A,P ) else replace [A]¬P by Reg(A,¬P ). end Notice that the action variable a of the successor state axiom is instantiated by the constant A. In our example, the regression of the subformula [shoot]¬Alive is ¬HasGun ∨ (shoot = shoot ∧ Loaded)∨ (shoot = strangle ∧ >) ∨ (¬Alive ∧ ¬⊥) This can be simplified to ¬HasGun∨Loaded∨¬Alive. Hence the result of the regression of Φ is ¬HasGun ∨ [load](¬HasGun ∨ Loaded ∨ ¬Alive). Each rewriting step thus eliminates a modal operator, and iterated application results in a formula without modal operators. If we iterate regression in our example, we first put the formula ¬HasGun ∨ [load](¬HasGun ∨ Loaded ∨ ¬Alive) 174 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 into normal form, obtaining ¬HasGun ∨ [load]¬HasGun ∨ [load]Loaded ∨ [load]¬Alive. The regression of subformula [load]¬HasGun is equivalent to ¬HasGun, that of subformula [load]Loaded to >, and that of [load]¬Alive to ¬Alive. We therefore obtain ¬HasGun ∨ ¬HasGun ∨ > ∨ ¬Alive, which is valid in classical propositional logic. Thus the original formula HasGun → [load][shoot]¬Alive can be deduced from Reiter's action theoryR. As regression is proved to be sound [REI 01, Theorem 4.5.2], checking validity of the original formula amounts to checking satisfiability of the regressed one in the initial state of the world: THEOREM 2. - R |=RT R Φ↔ REG(Φ). COROLLARY 3. - R |=RT R Φ if and only if REG(Φ) is valid in Classical Propositional Logic. The rest of the paper explores whether regression can be performed in a simpler framework, in particular without quantifying over actions. 6. De Giacomo and Lenzerini's encoding into PDL Reiter's Situation Calculus based solution has raised the natural question of at what extent it could be possible to do the same in dynamic logic. Given the expressiveness limitations of the latter (originally it did not allow for quantification over actions), many researchers [ZHA 01] have turned to other ways of facing the problems in the area. There has been others [GIA 95], however, who have tried on the first steps in that direction. De Giacomo and Lenzerini have expressed Reiter's solution in a slightly modified version of PDL. This is what we take up in this section. Here we simplify their account a bit and suppose that the set of atomic actions is the finite ACT = {A1, A2, . . . , An}. Then their approach can be said to have the following ingredients (α, β, . . . denote complex actions): – Nondeterministic choice α ∪ β; – Converse α−; – A particular nondeterministic atomic action any that can be thought of as the nondeterministic composition of all atomic actions of ACT: any = A1∪A2∪. . .∪An; – Complement ¬α w.r.t. any, where α = B1 ∪ . . .∪Bm for some B1, . . . , Bm ∈ ACT. Moreover it is supposed that the past is deterministic, as expressed by the logical axiom ¬[any−]¬Φ→ [any−]Φ. Regression in Modal Logic 175 Consider our example theory. Its representation in De Giacomo and Lenzerini's framework is: [any](¬Alive→ 〈any−〉¬Alive ∨ 〈shoot−〉Loaded ∨ 〈strangle−〉>) [any](Alive→ 〈any−〉Alive) Just as for PDL, reasoning in De Giacomo and Lenzerini's logical framework is EXPTIME-complete. While their encoding certainly preserves the spirit of Reiter's successor state axioms, they did not give the counterpart of Reiter's regression, and hence did not investigate whether reasoning for syntactically restricted theories is "cheaper" than EXPTIME. In the next section we show how this can be simulated without quantification in a simple modal logic of actions augmented by a dependence relation. 7. Solving the frame problem without quantification 7.1. Adding dependence information to PDL In [CAS 99] we have augmented a very simple version of PDL (basically multimodal K) with metalogical causal information represented by a dependence relation ; between actions and literals. A;L means "action A may cause literal L". The nonexistence of such a A;L in ; (noted A6;L) means that "L will never get true due to A".3 A;P is just another way of writing down thatA ∈ Cause+(P ), andA;¬P that A ∈ Cause−(P ). Suppose ; is given. Semantically, if I(A) is the accessibility relation associated to action A, the relation ; constrains possible worlds models in the following way: – if A6;P and w′ ∈ (I(A))(w) and w 6∈ I(P ) then w′ 6∈ I(P ); – if A6;¬P and w′ ∈ (I(A))(w) and w ∈ I(P ) then w′ ∈ I(P ). The resulting class of models is called LAP;. We note DLAP; the class of LAP;-models whose accessibility relations are deterministic. It has been shown in [CAS 99] that the validities ofLAP; are completely axiomatized by the following set of logical axioms: 1) Some axiomatization of classical logic; 2) [A]Φ ∧ [A](Φ→ Ψ)→ [A]Ψ; 3) ¬L→ [A]¬L if A6;L. 3. In [CAS 99] the language moreover contained an S4 modal operator 2 which implies all action operators [A]. Laws were prefixed with 2, e.g. 2(Loaded → [shoot]¬Alive). Here we shall achieve the same thing by viewing action laws as global axioms. 176 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 plus the Modus Ponens and the necessitation rule. It has moreover been shown that LAP; is decidable and EXPTIME-complete, and a tableau theorem proving method has been given. 7.2. Solving the frame problem in LAP; Suppose all the ingredients Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ), Cond −(A,P ) are given, and let us make the completeness assumptions as introduced in Section 3. We define a dependence relation and a set of global axioms S as follows: – for every Ai ∈ Cause +(P ) we put Ai;P , and for everyBj ∈ Cause −(P ) we put Bj;¬P ; – for every A ∈ ACT , add an executability axiom to S: Poss(A)↔ ¬[A]⊥ (4) – for every P ∈ PRP and every Ai ∈ Cause +(P ) add two effect axioms to S: Cond +(Ai, P )→ [Ai]P (5) (¬Cond+(Ai, P ) ∧ ¬P )→ [Ai]¬P (6) – for every P ∈ PRP and every Bj ∈ Cause −(P ) add two effect axioms to S: Cond −(Bj , P )→ [Bj ]¬P (7) (¬Cond−(Bj , P ) ∧ P )→ [Bj ]P (8) Note that these axioms do not resemble successor state axioms. They nevertheless validate the same regression principle as in Reiter's framework, as it will be shown in the sequel. A point that bears noting is that our representation indeed counts as a solution to the frame problem: the sets ; and S are both "small" (in the sense that they are much smaller than card(PRP)× card(ACT )), and contain no frame axioms. Now we turn to an important result: THEOREM 4. - Let ; and S be obtained from given sets Poss(A), Cause+(P ), Cause −(P ), Cond+(A,P ) and Cond−(A,P ). Then the following equivalences are logical consequences of S in DLAP;. 1) [A]P ↔ ¬Poss(A) ∨ P , if A6;P and A6;¬P ; 2) [A]P ↔ ¬Poss(A) ∨ (P ∧ ¬Cond−(A,P )), if A6;P and A;¬P ; 3) [A]P ↔ ¬Poss(A) ∨ Cond+(A,P ) ∨ P , if A;P and A6;¬P ; 4) [A]P ↔ ¬Poss(A) ∨ Cond+(A,P ) ∨ (P ∧ ¬Cond−(A,P )), if A;P and A;¬P . Regression in Modal Logic 177 PROOF. - Proving (1): (→): We are about to prove [A]P ∧ ¬P → ¬Poss(A). 1. ¬P → [A]¬P , from the hypothesis A6;P 2. [A]P ∧ ¬P → [A]P ∧ [A]¬P , from 1. by classical logic 3. [A]P ∧ [A]¬P → [A]⊥, by K and classical logic 4. [A]P ∧ ¬P → [A](P ∧ ¬P ), from 2. and 3. by syllogism 5. [A](P ∧ ¬P )→ [A]⊥, by classical logic 6. [A]P ∧ ¬P → [A]⊥, by syllogism on 4. and 5. 7. [A]⊥ → ¬Poss(A), from global axiom (4) 8. [A]P ∧ [A]¬P → ¬Poss(A), from 3. and 7. by classical logic 9. [A]P ∧ ¬P → ¬Poss(A), from 2. and 8. by classical logic (←): We now prove ¬Poss(A) ∨ P → [A]P . 1. P → [A]P , from the hypothesisA6;¬P 2. ¬Poss(A)→ [A]⊥, from global axiom (4) 3. [A]⊥ → [A]P , by K and classical logic 4. ¬Poss(A)→ [A]P , from 2. and 3. by classical logic 5. ¬Poss(A) ∨ P → [A]P , from 1. and 4. by classical logic Proving (2): (→): Let's show [A]P ∧¬P → ¬Poss(A) and [A]P ∧Cond−(A,P )→ ¬Poss(A). 1. ¬P → [A]¬P , from the hypothesis A6;P 2. [A]P ∧ ¬P → [A]P ∧ [A]¬P , from 1. by classical logic 3. [A]P ∧ [A]¬P → [A]⊥, by K and classical logic 4. [A]⊥ → ¬Poss(A), from global axiom (4) 5. [A]P ∧ [A]¬P → ¬Poss(A), from 3. and 4. by classical logic 6. [A]P ∧ ¬P → ¬Poss(A), from 2. and 5. by classical logic 178 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 7. Cond−(A,P )→ [A]¬P , by global axiom (7) 8. [A]P ∧ Cond−(A,P )→ [A]P ∧ [A]¬P , from 7. by classical logic 9. [A]P ∧ Cond−(A,P )→ [A]⊥, from 8. and 3. by classical logic 10. [A]P ∧ Cond−(A,P )→ ¬Poss(A), from 9. and 4. by classical logic (←): We are going to prove ¬Poss(A) ∨ (P ∧ ¬Cond−(A,P ))→ [A]P . 1. ¬Poss(A)→ [A]⊥, from global axiom (4) 2. [A]⊥ → [A]P , by K and classical logic 3. ¬Poss(A)→ [A]P , from 1. and 2. by classical logic 4. (P ∧ ¬Cond−(A,P ))→ [A]P , from global axiom (8) 5. ¬Poss(A) ∨ (P ∧ ¬Cond−(A,P ))→ [A]P , from 3. and 4. by classical logic Proving (3): (→): We will prove [A]P ∧ ¬Cond+(A,P ) ∧ ¬P → ¬Poss(A). 1. ¬Cond+(A,P ) ∧ ¬P → [A]¬P , by global axiom (6) 2. [A]P ∧ ¬Cond+(A,P ) ∧ ¬P → [A]P ∧ [A]¬P , from 1. by classical logic 3. [A]P ∧ [A]¬P → [A]⊥, by K and classical logic 4. [A]P ∧ ¬Cond+(A,P ) ∧ ¬P → [A]⊥, from 2. and 3. by classical logic 5. [A]⊥ → ¬Poss(A), from global axiom (4) 6. [A]P ∧ ¬Cond+(A,P ) ∧ ¬P → ¬Poss(A), from 4. and 5. by classical logic (←): We are about to prove ¬Poss(A) ∨ Cond+(A,P ) ∨ P → [A]P 1. ¬Poss(A)→ [A]⊥, from global axiom (4) 2. [A]⊥ → [A]P , by K and classical logic 3. ¬Poss(A)→ [A]P , from 1. and 2. by classical logic 4. P → [A]P , by hypothesis A6;¬P 5. Cond+(A,P )→ [A]P , from global axiom (5) 6. ¬Poss(A) ∨ Cond+(A,P ) ∨ P → [A]P , from 3., 4. and 5. by classical logic Regression in Modal Logic 179 Proving (4): (→): We prove [A]P ∧ ¬Cond+(A,P ) ∧ ¬(P ∧ ¬Cond−(A,P ))→ ¬Poss(A) 1. ¬Cond+(A,P ) ∧ ¬P → [A]¬P , from global axiom (6) 2. [A]P ∧ ¬Cond+(A,P ) ∧ ¬P → [A]P ∧ [A]¬P , from 1. by classical logic 3. Cond−(A,P )→ [A]¬P , by global axiom (7) 4. [A]P ∧¬Cond+(A,P )∧Cond−(A,P )→ [A]P ∧¬Cond+(A,P )∧ [A]¬P , from 3. by classical logic 5. [A]P ∧ ¬Cond+(A,P ) ∧ [A]¬P → [A]P ∧ [A]¬P , by classical logic 6. [A]P ∧ ¬Cond+(A,P ) ∧ Cond−(A,P ) → [A]P ∧ [A]¬P , from 4. and 5. by classical logic 7. [A]P ∧ ¬Cond+(A,P ) ∧ ¬P ∨ [A]P ∧ ¬Cond+(A,P ) ∧ Cond−(A,P ) → [A]P ∧ [A]¬P , from 2. and 4. by classical logic 8. [A]P ∧ ¬Cond+(A,P ) ∧ ¬(P ∧ ¬Cond−(A,P ))→ [A]P ∧ [A]¬P , from 7. by classical logic 9. [A]P ∧ [A]¬P → [A]⊥, by K and classical logic 10. [A]P ∧¬Cond+(A,P )∧¬(P ∧¬Cond−(A,P ))→ [A]⊥, from 8. and 9. by classical logic 11. [A]⊥ → ¬Poss(A), from global axiom (4) 12. [A]P ∧ ¬Cond+(A,P ) ∧ ¬(P ∧ ¬Cond−(A,P )) → ¬Poss(A), from 10. and 11. by classical logic (←): We will prove ¬Poss(A) ∨ Cond+(A,P ) ∨ (P ∧ ¬Cond−(A,P ))→ [A]P 1. ¬Poss(A)→ [A]⊥, from global axiom (4) 2. [A]⊥ → [A]P , by K and classical logic 3. ¬Poss(A)→ [A]P , from 1. and 2. by classical logic 4. Cond+(A,P )→ [A]P , from global axiom (5) 5. (P ∧ ¬Cond−(A,P ))→ [A]P , by global axiom (8) 6. ¬Poss(A)∨Cond+(A,P )∨(P ∧¬Cond−(A,P ))→ [A]P , from 3., 4. and 5. by classical logic 180 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 8. Regression in DLAP; Here is a regression algorithm for DLAP;. Suppose Φ is a complex formula without quantification and equality, such as HasGun → [load][shoot]¬Alive. Let us consider Cond(A,L) = Cond+(A,P ) if L = P , and Cond (A,L) = Cond−(A,P ) if L = ¬P . ALGORITHM 5 (REGRESSION WITH DEPENDENCE). - input: a formula without variables Φ. Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ) and Cond−(A,P ). output: a classical formula REG(Φ). begin while Φ is not classical put Φ in normal form choose some subformula [A]L, where L is a literal case A6;L and A6;¬L then replace [A]L by ¬Poss(A) ∨ L case A6;L and A;¬L then replace [A]L by ¬Poss(A) ∨ (L ∧ ¬Cond (A,¬L)) caseA;L andA6;¬L then replace [A]L by ¬Poss(A)∨Cond (A,L)∨L case A;L and A;¬L then replace [A]L by ¬Poss(A) ∨ Cond(A,L) ∨ (L ∧ ¬Cond (A,¬L)) end In our example, the regression of [shoot]¬Alive is ¬HasGun ∨ Loaded ∨ ¬Alive. Hence the result of the regression step is HasGun → [load](¬HasGun ∨ Loaded ∨ ¬Alive). Putting this into normal form using (3) we obtain the formula HasGun → ([load]¬HasGun∨[load]Loaded∨[load]¬Alive). The regression of [load]¬HasGun is ¬HasGun, that of [load]Loaded is >, and that of [load]¬Alive is ¬Alive. We therefore obtain HasGun→ (¬HasGun∨>∨¬Alive), which is valid in classical propositional logic. THEOREM 6 (DECIDABILITY, SOUNDNESS AND COMPLETENESS). - Suppose S and ; obtained from Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ) and Cond −(A,P ) as described in Section 7.2. Let Φ be an input formula without quantifiers, action variables, and equality. Then, Algorithm 5 terminates returning a classical formula φ and S|=DLAP;Φ↔ φ. PROOF. - Let Φ be an input formula. Termination is straightforward, as each step of the algorithm eliminates exactly one modal operator. Soundness and completeness Regression in Modal Logic 181 are also immediate: after putting formula Φ in normal form, it will be made of conjunctions/disjunctions of modal subformulas. In this case, the equivalence between Φ and φ follows from the ones given in theorem 4 together with the rule of substitution of equivalences (which is valid in DLAP;). For our example, this means that HasGun → [load][shoot]¬Alive can be deduced with our action theory S and dependence relation ; because its regression is valid in classical logic. Hence, modulo the equality predicate, we obtain the same result as for Reiter's regression algorithm in the case of our example. This generalizes: a close look at the two algorithms shows that if both our S and ; and Reiter's R are obtained from the same Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ), Cond−(A,P ), then the results are logically equivalent. It follows thus that whenever Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ), Cond −(A,P ) are given, and the completeness assumptions can be made, then Reiter's formulation in terms of successor state axioms and ours in terms of effect axioms and dependence do the same job in their respective logical basis: COROLLARY 7. - Let the sets Poss(A), Cause+(P ), Cause−(P ), Cond+(A,P ), Cond −(A,P ) be given. Let R be a Reiter theory obtained from them as described in Section 4. Let ; and S be obtained from them as described in Section 7.2. Let Φ be a complex formula without quantification and equality. Then R |=RT R Φ iff S|=DLAP;Φ. 9. The frame problem for knowledge 9.1. Sensing actions and knowledge Reiter's framework does not account for actions which have no effect on the "real" world, but only on the agents' knowledge. Such actions are close to test actions of dynamic logic. In order to express the effects of sensing actions we need a modal operator of knowledge 2. The logic of 2 is S5. The dual of 2 is noted 3. The extension of Reiter's solution to knowledge and sensing actions has been studied by Scherl and Levesque [SCH 93]. They make some hypotheses about actions and their perception by the agent that permit to simplify the theory. 9.1.1. Public action First, they suppose that the agent perceives action occurrences completely and correctly. For example whenever shooting takes place the agent is aware of that, and whenever the agent believes shooting has taken place then indeed such an action has occurred. (One might imagine that action occurrences are publicly announced to all agents.) 182 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 9.1.2. Action laws known Second, they suppose that the agent knows the laws governing the actions. Hence the agent knows that after strangling the effect always is ¬Alive, etc. 9.1.3. Non-informative actions We finally make a third hypothesis that is not made by Scherl and Levesque, but which simplifies exposition without too much loss of generality [HER 00]. We shall suppose henceforth that all actions are non-informative. Non-informative actions are actions which are not observed by the agent beyond their mere occurrence. Upon learning that such an action has occurred the agent updates his belief state: he computes the new belief state from the previous one and his knowledge about the action laws. Hence the new belief state neither depends on the state of the world before the action occurrence, nor on the state of the world after the action occurrence. In our example the shoot action is non informative. If the agent learns that the shooting action has been executed then he does not learn whether the victim died or not: if both Loaded and ¬Loaded were possible for the agent before, then afterwards he envisages both possible outcomes. Nevertheless, by learning that shoot occurred the agent learns that HasGun was true before the action. Clearly, the action of observing the outcome of the shoot action is informative: the new belief state depends on the truth value of ¬Alive in the real world. Other examples of informative actions are that of looking up a phone number, testing if a proposition is true, telling whether a proposition is true, etc. Nevertheless, the agent is not disconnected from the world: he may learn that some proposition is true (i.e. that some action of observing that some proposition has some value has occurred). For example, when he learns that it has been observed that the victim is dead (i.e. he learns that the action of observing ¬Alive has been executed) then he is able to update his belief state accordingly. Indeed, the observe actions are non-informative according to our definition: when the agent learns that φ has been observed then he is able to update his belief state accordingly, and there is no need to further observe the world. Other examples of non-informative actions are that of learning that the phone number of another agent is N , testing that a proposition is true (in the sense of dynamic logic tests), telling that a proposition is true, etc. 9.1.4. A successor state axiom Under the hypotheses we have made, the following logical axiom is reasonable: [A]2Φ↔ ([A]⊥ ∨2[A]Φ) From the left to the right, this corresponds to a "no forgetting" principle, while the right-to-left direction expresses a "no learning" principle. Regression in Modal Logic 183 Such an axiom has been called a successor state axiom by Scherl and Levesque.4 It permits to solve what they have called the frame problem for knowledge. First, note that because actions are supposed to be deterministic, this axiom allows to deduce [A]3Φ↔ ([A]⊥ ∨3¬[A]¬Φ). Now these two principles enable regression by allowing for the elimination of 2 and 3 operators from the scope of action operators. When all such epistemic operators have been moved outward, Reiter's regression can be applied to the remaining non-epistemic formula, resulting in a modality-free formula of classical propositional logic. As a whole, the resulting formula only contains epistemic operators, but no action operators. To sum it up, to establish whether a complex formula Φ follows from a domain description amounts to – move all [A] operators inwards, then – eliminate all [A] operators by regression, and finally – check whether the resulting formula Ψ is a theorem of S5. 10. Concluding remarks In this paper we have presented a purely propositional framework for reasoning about actions in modal logic within which Reiter's regression technique can be applied. We have thus shown that regression does not necessarily build on successor state axioms as in Reiter's original theory, which involves quantification. We have also seen how the ideas here developed could be extended and applied in reasoning about knowledge. As we have presented it here, Reiter's solution is very constrained. In particular actions must be deterministic and without indirect effects. Reiter has proposed [REI 01] to implement nondeterministic actions by means of an operator of nondeterministic composition of deterministic atomic actions similar to that of dynamic logic. For example, the action toss of tossing a coin can be thought of as the nondeterministic choice tossHeads∪ tossTails between tossHeads and tossTails, whose respective effects are Heads and Tails. [tossHeads ∪ tossTails]Φ is defined to be an abbreviation of [tossHeads]Φ ∧ [tossTails]Φ. Such a solution transfers straightforwardly to our modal logic. But our framework also offers a more straightforward way of dealing with actions with indeterminate effects: we can drop the hypothesis that for every A ∈ Cause+(P ) the condition Cond+(A,P ) is defined. For example, 4. Their successor state axiom contains supplementary conditions in order to account for the informative part of actions. 184 Journal of Applied Non-Classical Logics. Volume 13 – n◦ 2/2003 although Cause+(Heads) = Cause+(Tails) = {toss}, there is no way of stating the exact conditions when heads or tails results from tossing. Reiter's solution supposes that domain descriptions only contain executability and effect laws. Thus it does not allow for static laws such as Walking → Alive. Such laws augment the effects of the shoot action: shooting not only has the (direct) effect ¬Alive, but also the (indirect) effect ¬Walking. Reiter and Lin [LIN 94] have proposed to "compile away" static laws in a mechanical way into effect laws (see also [MCI 98]). Again, this transfers straightforwardly to our modal logic. Nevertheless, the most challenging continuation of our work is the direct integration of socalled state constraints into the framework (instead of compiling them away as done by Lin and Reiter). But things get much harder in this case, all the more in [CAS 02] we have claimed that up to now there is no satisfactory framework allowing for actions with both indirect and indeterminate effects. We plan to pursue future works analyzing to at what extent the results here presented could be generalized to Lin's [LIN 95, LIN 96] approach in the case of stratified action theories. Acknowledgements Ivan Varzinczak has been supported by a fellowship from the Federative Republic of Brazil. Grant: BEX 1389/01-7 (CAPES). 11. References [CAS 99] CASTILHO M. A., GASQUET O., HERZIG A., "Formalizing Action and Change in Modal Logic I: the frame problem", J. of Logic and Computation, vol. 9, num. 5, 1999, p. 701–735. [CAS 02] CASTILHO M. A., HERZIG A., VARZINCZAK I., "It depends on the context! 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