1 (*<*)theory PDL imports Base begin(*>*)
3 subsection{*Propositional Dynamic Logic --- PDL*}
6 The formulae of PDL are built up from atomic propositions via
7 negation and conjunction and the two temporal
8 connectives @{text AX} and @{text EF}\@. Since formulae are essentially
9 syntax trees, they are naturally modelled as a datatype:%
10 \footnote{The customary definition of PDL
11 \cite{HarelKT-DL} looks quite different from ours, but the two are easily
12 shown to be equivalent.}
15 datatype formula = Atom "atom"
22 This resembles the boolean expression case study in
24 A validity relation between states and formulae specifies the semantics.
25 The syntax annotation allows us to write @{text"s \<Turnstile> f"} instead of
26 \hbox{@{text"valid s f"}}. The definition is by recursion over the syntax:
29 primrec valid :: "state \<Rightarrow> formula \<Rightarrow> bool" ("(_ \<Turnstile> _)" [80,80] 80)
31 "s \<Turnstile> Atom a = (a \<in> L s)" |
32 "s \<Turnstile> Neg f = (\<not>(s \<Turnstile> f))" |
33 "s \<Turnstile> And f g = (s \<Turnstile> f \<and> s \<Turnstile> g)" |
34 "s \<Turnstile> AX f = (\<forall>t. (s,t) \<in> M \<longrightarrow> t \<Turnstile> f)" |
35 "s \<Turnstile> EF f = (\<exists>t. (s,t) \<in> M\<^sup>* \<and> t \<Turnstile> f)"
38 The first three equations should be self-explanatory. The temporal formula
39 @{term"AX f"} means that @{term f} is true in \emph{A}ll ne\emph{X}t states whereas
40 @{term"EF f"} means that there \emph{E}xists some \emph{F}uture state in which @{term f} is
41 true. The future is expressed via @{text"\<^sup>*"}, the reflexive transitive
42 closure. Because of reflexivity, the future includes the present.
44 Now we come to the model checker itself. It maps a formula into the
45 set of states where the formula is true. It too is defined by
46 recursion over the syntax: *}
48 primrec mc :: "formula \<Rightarrow> state set" where
49 "mc(Atom a) = {s. a \<in> L s}" |
51 "mc(And f g) = mc f \<inter> mc g" |
52 "mc(AX f) = {s. \<forall>t. (s,t) \<in> M \<longrightarrow> t \<in> mc f}" |
53 "mc(EF f) = lfp(\<lambda>T. mc f \<union> (M\<inverse> `` T))"
56 Only the equation for @{term EF} deserves some comments. Remember that the
57 postfix @{text"\<inverse>"} and the infix @{text"``"} are predefined and denote the
58 converse of a relation and the image of a set under a relation. Thus
59 @{term "M\<inverse> `` T"} is the set of all predecessors of @{term T} and the least
60 fixed point (@{term lfp}) of @{term"\<lambda>T. mc f \<union> M\<inverse> `` T"} is the least set
61 @{term T} containing @{term"mc f"} and all predecessors of @{term T}. If you
62 find it hard to see that @{term"mc(EF f)"} contains exactly those states from
63 which there is a path to a state where @{term f} is true, do not worry --- this
64 will be proved in a moment.
66 First we prove monotonicity of the function inside @{term lfp}
67 in order to make sure it really has a least fixed point.
70 lemma mono_ef: "mono(\<lambda>T. A \<union> (M\<inverse> `` T))"
76 Now we can relate model checking and semantics. For the @{text EF} case we need
81 "lfp(\<lambda>T. A \<union> (M\<inverse> `` T)) = {s. \<exists>t. (s,t) \<in> M\<^sup>* \<and> t \<in> A}"
84 The equality is proved in the canonical fashion by proving that each set
85 includes the other; the inclusion is shown pointwise:
90 apply(simp)(*<*)apply(rename_tac s)(*>*)
93 Simplification leaves us with the following first subgoal
94 @{subgoals[display,indent=0,goals_limit=1]}
95 which is proved by @{term lfp}-induction:
98 apply(erule lfp_induct_set)
101 (*pr(latex xsymbols symbols);*)
103 Having disposed of the monotonicity subgoal,
104 simplification leaves us with the following goal:
106 \ {\isadigit{1}}{\isachardot}\ {\isasymAnd}x{\isachardot}\ x\ {\isasymin}\ A\ {\isasymor}\isanewline
107 \ \ \ \ \ \ \ \ \ x\ {\isasymin}\ M{\isasyminverse}\ {\isacharbackquote}{\isacharbackquote}\ {\isacharparenleft}lfp\ {\isacharparenleft}\dots{\isacharparenright}\ {\isasyminter}\ {\isacharbraceleft}x{\isachardot}\ {\isasymexists}t{\isachardot}\ {\isacharparenleft}x{\isacharcomma}\ t{\isacharparenright}\ {\isasymin}\ M\isactrlsup {\isacharasterisk}\ {\isasymand}\ t\ {\isasymin}\ A{\isacharbraceright}{\isacharparenright}\isanewline
108 \ \ \ \ \ \ \ \ {\isasymLongrightarrow}\ {\isasymexists}t{\isachardot}\ {\isacharparenleft}x{\isacharcomma}\ t{\isacharparenright}\ {\isasymin}\ M\isactrlsup {\isacharasterisk}\ {\isasymand}\ t\ {\isasymin}\ A
110 It is proved by @{text blast}, using the transitivity of
111 \isa{M\isactrlsup {\isacharasterisk}}.
114 apply(blast intro: rtrancl_trans)
117 We now return to the second set inclusion subgoal, which is again proved
125 After simplification and clarification we are left with
126 @{subgoals[display,indent=0,goals_limit=1]}
127 This goal is proved by induction on @{term"(s,t)\<in>M\<^sup>*"}. But since the model
128 checker works backwards (from @{term t} to @{term s}), we cannot use the
129 induction theorem @{thm[source]rtrancl_induct}: it works in the
130 forward direction. Fortunately the converse induction theorem
131 @{thm[source]converse_rtrancl_induct} already exists:
132 @{thm[display,margin=60]converse_rtrancl_induct[no_vars]}
133 It says that if @{prop"(a,b):r\<^sup>*"} and we know @{prop"P b"} then we can infer
134 @{prop"P a"} provided each step backwards from a predecessor @{term z} of
135 @{term b} preserves @{term P}.
138 apply(erule converse_rtrancl_induct)
142 @{subgoals[display,indent=0,goals_limit=1]}
143 is solved by unrolling @{term lfp} once
146 apply(subst lfp_unfold[OF mono_ef])
149 @{subgoals[display,indent=0,goals_limit=1]}
150 and disposing of the resulting trivial subgoal automatically:
156 The proof of the induction step is identical to the one for the base case:
159 apply(subst lfp_unfold[OF mono_ef])
164 The main theorem is proved in the familiar manner: induction followed by
165 @{text auto} augmented with the lemma as a simplification rule.
168 theorem "mc f = {s. s \<Turnstile> f}"
170 apply(auto simp add: EF_lemma)
175 @{term AX} has a dual operator @{term EN}
176 (``there exists a next state such that'')%
177 \footnote{We cannot use the customary @{text EX}: it is reserved
178 as the \textsc{ascii}-equivalent of @{text"\<exists>"}.}
179 with the intended semantics
180 @{prop[display]"(s \<Turnstile> EN f) = (EX t. (s,t) : M & t \<Turnstile> f)"}
181 Fortunately, @{term"EN f"} can already be expressed as a PDL formula. How?
183 Show that the semantics for @{term EF} satisfies the following recursion equation:
184 @{prop[display]"(s \<Turnstile> EF f) = (s \<Turnstile> f | s \<Turnstile> EN(EF f))"}
189 theorem main: "mc f = {s. s \<Turnstile> f}"
191 apply(auto simp add: EF_lemma)
194 lemma aux: "s \<Turnstile> f = (s : mc f)"
195 apply(simp add: main)
198 lemma "(s \<Turnstile> EF f) = (s \<Turnstile> f | s \<Turnstile> Neg(AX(Neg(EF f))))"
199 apply(simp only: aux)
201 apply(subst lfp_unfold[OF mono_ef], fast)