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(* Author: Tobias Nipkow *)


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header "Live Variable Analysis"


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theory Live imports Vars Big_Step


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begin


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subsection "Liveness Analysis"


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fun L :: "com \<Rightarrow> name set \<Rightarrow> name set" where


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"L SKIP X = X" 


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"L (x ::= a) X = X{x} \<union> vars a" 


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"L (c\<^isub>1; c\<^isub>2) X = (L c\<^isub>1 \<circ> L c\<^isub>2) X" 


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"L (IF b THEN c\<^isub>1 ELSE c\<^isub>2) X = vars b \<union> L c\<^isub>1 X \<union> L c\<^isub>2 X" 


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"L (WHILE b DO c) X = vars b \<union> X \<union> L c X"


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value "show (L (''y'' ::= V ''z''; ''x'' ::= Plus (V ''y'') (V ''z'')) {''x''})"


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value "show (L (WHILE Less (V ''x'') (V ''x'') DO ''y'' ::= V ''z'') {''x''})"


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fun "kill" :: "com \<Rightarrow> name set" where


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"kill SKIP = {}" 


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"kill (x ::= a) = {x}" 


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"kill (c\<^isub>1; c\<^isub>2) = kill c\<^isub>1 \<union> kill c\<^isub>2" 


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"kill (IF b THEN c\<^isub>1 ELSE c\<^isub>2) = kill c\<^isub>1 \<inter> kill c\<^isub>2" 


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"kill (WHILE b DO c) = {}"


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fun gen :: "com \<Rightarrow> name set" where


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"gen SKIP = {}" 


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"gen (x ::= a) = vars a" 


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"gen (c\<^isub>1; c\<^isub>2) = gen c\<^isub>1 \<union> (gen c\<^isub>2  kill c\<^isub>1)" 


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"gen (IF b THEN c\<^isub>1 ELSE c\<^isub>2) = vars b \<union> gen c\<^isub>1 \<union> gen c\<^isub>2" 


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"gen (WHILE b DO c) = vars b \<union> gen c"


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lemma L_gen_kill: "L c X = (X  kill c) \<union> gen c"


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by(induct c arbitrary:X) auto


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lemma L_While_subset: "L c (L (WHILE b DO c) X) \<subseteq> L (WHILE b DO c) X"


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by(auto simp add:L_gen_kill)


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subsection "Soundness"


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theorem L_sound:


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"(c,s) \<Rightarrow> s' \<Longrightarrow> s = t on L c X \<Longrightarrow>


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\<exists> t'. (c,t) \<Rightarrow> t' & s' = t' on X"

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proof (induction arbitrary: X t rule: big_step_induct)

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case Skip then show ?case by auto


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next


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case Assign then show ?case


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by (auto simp: ball_Un)


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next


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case (Semi c1 s1 s2 c2 s3 X t1)

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from Semi.IH(1) Semi.prems obtain t2 where

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t12: "(c1, t1) \<Rightarrow> t2" and s2t2: "s2 = t2 on L c2 X"


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by simp blast

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from Semi.IH(2)[OF s2t2] obtain t3 where

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t23: "(c2, t2) \<Rightarrow> t3" and s3t3: "s3 = t3 on X"


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by auto


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show ?case using t12 t23 s3t3 by auto


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next


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case (IfTrue b s c1 s' c2)


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hence "s = t on vars b" "s = t on L c1 X" by auto


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from bval_eq_if_eq_on_vars[OF this(1)] IfTrue(1) have "bval b t" by simp


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from IfTrue(3)[OF `s = t on L c1 X`] obtain t' where


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"(c1, t) \<Rightarrow> t'" "s' = t' on X" by auto


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thus ?case using `bval b t` by auto


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next


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case (IfFalse b s c2 s' c1)


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hence "s = t on vars b" "s = t on L c2 X" by auto


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from bval_eq_if_eq_on_vars[OF this(1)] IfFalse(1) have "~bval b t" by simp


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from IfFalse(3)[OF `s = t on L c2 X`] obtain t' where


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"(c2, t) \<Rightarrow> t'" "s' = t' on X" by auto


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thus ?case using `~bval b t` by auto


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next


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case (WhileFalse b s c)


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hence "~ bval b t" by (auto simp: ball_Un) (metis bval_eq_if_eq_on_vars)


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thus ?case using WhileFalse(2) by auto


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next


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case (WhileTrue b s1 c s2 s3 X t1)


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let ?w = "WHILE b DO c"


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from `bval b s1` WhileTrue(6) have "bval b t1"


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by (auto simp: ball_Un) (metis bval_eq_if_eq_on_vars)


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have "s1 = t1 on L c (L ?w X)" using L_While_subset WhileTrue.prems


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by (blast)

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from WhileTrue.IH(1)[OF this] obtain t2 where

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"(c, t1) \<Rightarrow> t2" "s2 = t2 on L ?w X" by auto

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from WhileTrue.IH(2)[OF this(2)] obtain t3 where "(?w,t2) \<Rightarrow> t3" "s3 = t3 on X"

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by auto


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with `bval b t1` `(c, t1) \<Rightarrow> t2` show ?case by auto


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qed


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subsection "Program Optimization"


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text{* Burying assignments to dead variables: *}


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fun bury :: "com \<Rightarrow> name set \<Rightarrow> com" where


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"bury SKIP X = SKIP" 


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"bury (x ::= a) X = (if x:X then x::= a else SKIP)" 


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"bury (c\<^isub>1; c\<^isub>2) X = (bury c\<^isub>1 (L c\<^isub>2 X); bury c\<^isub>2 X)" 


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"bury (IF b THEN c\<^isub>1 ELSE c\<^isub>2) X = IF b THEN bury c\<^isub>1 X ELSE bury c\<^isub>2 X" 


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"bury (WHILE b DO c) X = WHILE b DO bury c (vars b \<union> X \<union> L c X)"


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text{* We could prove the analogous lemma to @{thm[source]L_sound}, and the


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proof would be very similar. However, we phrase it as a semantics


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preservation property: *}


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theorem bury_sound:


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"(c,s) \<Rightarrow> s' \<Longrightarrow> s = t on L c X \<Longrightarrow>


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\<exists> t'. (bury c X,t) \<Rightarrow> t' & s' = t' on X"

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proof (induction arbitrary: X t rule: big_step_induct)

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case Skip then show ?case by auto


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next


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case Assign then show ?case


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by (auto simp: ball_Un)


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next


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case (Semi c1 s1 s2 c2 s3 X t1)

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from Semi.IH(1) Semi.prems obtain t2 where

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t12: "(bury c1 (L c2 X), t1) \<Rightarrow> t2" and s2t2: "s2 = t2 on L c2 X"


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by simp blast

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from Semi.IH(2)[OF s2t2] obtain t3 where

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t23: "(bury c2 X, t2) \<Rightarrow> t3" and s3t3: "s3 = t3 on X"


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by auto


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show ?case using t12 t23 s3t3 by auto


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next


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case (IfTrue b s c1 s' c2)


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hence "s = t on vars b" "s = t on L c1 X" by auto


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from bval_eq_if_eq_on_vars[OF this(1)] IfTrue(1) have "bval b t" by simp


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from IfTrue(3)[OF `s = t on L c1 X`] obtain t' where


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"(bury c1 X, t) \<Rightarrow> t'" "s' =t' on X" by auto


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thus ?case using `bval b t` by auto


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next


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case (IfFalse b s c2 s' c1)


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hence "s = t on vars b" "s = t on L c2 X" by auto


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from bval_eq_if_eq_on_vars[OF this(1)] IfFalse(1) have "~bval b t" by simp


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from IfFalse(3)[OF `s = t on L c2 X`] obtain t' where


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"(bury c2 X, t) \<Rightarrow> t'" "s' = t' on X" by auto


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thus ?case using `~bval b t` by auto


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next


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case (WhileFalse b s c)


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hence "~ bval b t" by (auto simp: ball_Un) (metis bval_eq_if_eq_on_vars)


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thus ?case using WhileFalse(2) by auto


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next


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case (WhileTrue b s1 c s2 s3 X t1)


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let ?w = "WHILE b DO c"


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from `bval b s1` WhileTrue(6) have "bval b t1"


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by (auto simp: ball_Un) (metis bval_eq_if_eq_on_vars)


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have "s1 = t1 on L c (L ?w X)"


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using L_While_subset WhileTrue.prems by blast

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from WhileTrue.IH(1)[OF this] obtain t2 where

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"(bury c (L ?w X), t1) \<Rightarrow> t2" "s2 = t2 on L ?w X" by auto

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from WhileTrue.IH(2)[OF this(2)] obtain t3

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where "(bury ?w X,t2) \<Rightarrow> t3" "s3 = t3 on X"


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by auto


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with `bval b t1` `(bury c (L ?w X), t1) \<Rightarrow> t2` show ?case by auto


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qed


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corollary final_bury_sound: "(c,s) \<Rightarrow> s' \<Longrightarrow> (bury c UNIV,s) \<Rightarrow> s'"


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using bury_sound[of c s s' UNIV]


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by (auto simp: fun_eq_iff[symmetric])


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text{* Now the opposite direction. *}


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lemma SKIP_bury[simp]:


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"SKIP = bury c X \<longleftrightarrow> c = SKIP  (EX x a. c = x::=a & x \<notin> X)"


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by (cases c) auto


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lemma Assign_bury[simp]: "x::=a = bury c X \<longleftrightarrow> c = x::=a & x : X"


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by (cases c) auto


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lemma Semi_bury[simp]: "bc\<^isub>1;bc\<^isub>2 = bury c X \<longleftrightarrow>


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(EX c\<^isub>1 c\<^isub>2. c = c\<^isub>1;c\<^isub>2 & bc\<^isub>2 = bury c\<^isub>2 X & bc\<^isub>1 = bury c\<^isub>1 (L c\<^isub>2 X))"


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by (cases c) auto


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lemma If_bury[simp]: "IF b THEN bc1 ELSE bc2 = bury c X \<longleftrightarrow>


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(EX c1 c2. c = IF b THEN c1 ELSE c2 &


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bc1 = bury c1 X & bc2 = bury c2 X)"


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by (cases c) auto


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lemma While_bury[simp]: "WHILE b DO bc' = bury c X \<longleftrightarrow>


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(EX c'. c = WHILE b DO c' & bc' = bury c' (vars b \<union> X \<union> L c X))"


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by (cases c) auto


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theorem bury_sound2:


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"(bury c X,s) \<Rightarrow> s' \<Longrightarrow> s = t on L c X \<Longrightarrow>


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\<exists> t'. (c,t) \<Rightarrow> t' & s' = t' on X"

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proof (induction "bury c X" s s' arbitrary: c X t rule: big_step_induct)

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case Skip then show ?case by auto


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next


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case Assign then show ?case


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by (auto simp: ball_Un)


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next


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case (Semi bc1 s1 s2 bc2 s3 c X t1)


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then obtain c1 c2 where c: "c = c1;c2"


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and bc2: "bc2 = bury c2 X" and bc1: "bc1 = bury c1 (L c2 X)" by auto

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note IH = Semi.hyps(2,4)


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from IH(1)[OF bc1, of t1] Semi.prems c obtain t2 where

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t12: "(c1, t1) \<Rightarrow> t2" and s2t2: "s2 = t2 on L c2 X" by auto

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from IH(2)[OF bc2 s2t2] obtain t3 where

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t23: "(c2, t2) \<Rightarrow> t3" and s3t3: "s3 = t3 on X"


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by auto


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show ?case using c t12 t23 s3t3 by auto


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next


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case (IfTrue b s bc1 s' bc2)


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then obtain c1 c2 where c: "c = IF b THEN c1 ELSE c2"


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and bc1: "bc1 = bury c1 X" and bc2: "bc2 = bury c2 X" by auto


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have "s = t on vars b" "s = t on L c1 X" using IfTrue.prems c by auto


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from bval_eq_if_eq_on_vars[OF this(1)] IfTrue(1) have "bval b t" by simp

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note IH = IfTrue.hyps(3)


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from IH[OF bc1 `s = t on L c1 X`] obtain t' where

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"(c1, t) \<Rightarrow> t'" "s' =t' on X" by auto


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thus ?case using c `bval b t` by auto


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next


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case (IfFalse b s bc2 s' bc1)


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then obtain c1 c2 where c: "c = IF b THEN c1 ELSE c2"


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and bc1: "bc1 = bury c1 X" and bc2: "bc2 = bury c2 X" by auto


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have "s = t on vars b" "s = t on L c2 X" using IfFalse.prems c by auto


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from bval_eq_if_eq_on_vars[OF this(1)] IfFalse(1) have "~bval b t" by simp

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note IH = IfFalse.hyps(3)


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from IH[OF bc2 `s = t on L c2 X`] obtain t' where

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"(c2, t) \<Rightarrow> t'" "s' =t' on X" by auto


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thus ?case using c `~bval b t` by auto


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next


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case (WhileFalse b s c)


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hence "~ bval b t" by (auto simp: ball_Un dest: bval_eq_if_eq_on_vars)


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thus ?case using WhileFalse by auto


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next


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case (WhileTrue b s1 bc' s2 s3 c X t1)


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then obtain c' where c: "c = WHILE b DO c'"


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and bc': "bc' = bury c' (vars b \<union> X \<union> L c' X)" by auto


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let ?w = "WHILE b DO c'"


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from `bval b s1` WhileTrue.prems c have "bval b t1"


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by (auto simp: ball_Un) (metis bval_eq_if_eq_on_vars)

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note IH = WhileTrue.hyps(3,5)

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have "s1 = t1 on L c' (L ?w X)"


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using L_While_subset WhileTrue.prems c by blast

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with IH(1)[OF bc', of t1] obtain t2 where

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"(c', t1) \<Rightarrow> t2" "s2 = t2 on L ?w X" by auto

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from IH(2)[OF WhileTrue.hyps(6), of t2] c this(2) obtain t3

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where "(?w,t2) \<Rightarrow> t3" "s3 = t3 on X"


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by auto


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with `bval b t1` `(c', t1) \<Rightarrow> t2` c show ?case by auto


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qed


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corollary final_bury_sound2: "(bury c UNIV,s) \<Rightarrow> s' \<Longrightarrow> (c,s) \<Rightarrow> s'"


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using bury_sound2[of c UNIV]


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by (auto simp: fun_eq_iff[symmetric])


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corollary bury_iff: "(bury c UNIV,s) \<Rightarrow> s' \<longleftrightarrow> (c,s) \<Rightarrow> s'"


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by(metis final_bury_sound final_bury_sound2)


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end
