src/HOL/Induct/QuoNestedDataType.thy
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(*  Title:      HOL/Induct/QuoNestedDataType
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   2004  University of Cambridge
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*)
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header{*Quotienting a Free Algebra Involving Nested Recursion*}
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theory QuoNestedDataType imports Main begin
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subsection{*Defining the Free Algebra*}
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text{*Messages with encryption and decryption as free constructors.*}
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datatype
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     freeExp = VAR  nat
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             | PLUS  freeExp freeExp
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             | FNCALL  nat "freeExp list"
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text{*The equivalence relation, which makes PLUS associative.*}
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text{*The first rule is the desired equation. The next three rules
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make the equations applicable to subterms. The last two rules are symmetry
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and transitivity.*}
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inductive_set
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  exprel :: "(freeExp * freeExp) set"
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  and exp_rel :: "[freeExp, freeExp] => bool"  (infixl "\<sim>" 50)
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  where
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    "X \<sim> Y == (X,Y) \<in> exprel"
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  | ASSOC: "PLUS X (PLUS Y Z) \<sim> PLUS (PLUS X Y) Z"
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  | VAR: "VAR N \<sim> VAR N"
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  | PLUS: "\<lbrakk>X \<sim> X'; Y \<sim> Y'\<rbrakk> \<Longrightarrow> PLUS X Y \<sim> PLUS X' Y'"
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  | FNCALL: "(Xs,Xs') \<in> listrel exprel \<Longrightarrow> FNCALL F Xs \<sim> FNCALL F Xs'"
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  | SYM:   "X \<sim> Y \<Longrightarrow> Y \<sim> X"
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  | TRANS: "\<lbrakk>X \<sim> Y; Y \<sim> Z\<rbrakk> \<Longrightarrow> X \<sim> Z"
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  monos listrel_mono
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text{*Proving that it is an equivalence relation*}
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lemma exprel_refl: "X \<sim> X"
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  and list_exprel_refl: "(Xs,Xs) \<in> listrel(exprel)"
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  by (induct X and Xs) (blast intro: exprel.intros listrel.intros)+
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theorem equiv_exprel: "equiv UNIV exprel"
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proof -
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  have "refl exprel" by (simp add: refl_on_def exprel_refl)
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  moreover have "sym exprel" by (simp add: sym_def, blast intro: exprel.SYM)
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  moreover have "trans exprel" by (simp add: trans_def, blast intro: exprel.TRANS)
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  ultimately show ?thesis by (simp add: equiv_def)
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qed
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theorem equiv_list_exprel: "equiv UNIV (listrel exprel)"
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  using equiv_listrel [OF equiv_exprel] by simp
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lemma FNCALL_Nil: "FNCALL F [] \<sim> FNCALL F []"
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apply (rule exprel.intros) 
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apply (rule listrel.intros) 
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done
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lemma FNCALL_Cons:
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     "\<lbrakk>X \<sim> X'; (Xs,Xs') \<in> listrel(exprel)\<rbrakk>
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      \<Longrightarrow> FNCALL F (X#Xs) \<sim> FNCALL F (X'#Xs')"
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by (blast intro: exprel.intros listrel.intros) 
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subsection{*Some Functions on the Free Algebra*}
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subsubsection{*The Set of Variables*}
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text{*A function to return the set of variables present in a message.  It will
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be lifted to the initial algrebra, to serve as an example of that process.
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Note that the "free" refers to the free datatype rather than to the concept
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of a free variable.*}
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primrec freevars :: "freeExp \<Rightarrow> nat set" 
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  and freevars_list :: "freeExp list \<Rightarrow> nat set" where
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  "freevars (VAR N) = {N}"
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| "freevars (PLUS X Y) = freevars X \<union> freevars Y"
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| "freevars (FNCALL F Xs) = freevars_list Xs"
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| "freevars_list [] = {}"
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| "freevars_list (X # Xs) = freevars X \<union> freevars_list Xs"
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text{*This theorem lets us prove that the vars function respects the
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equivalence relation.  It also helps us prove that Variable
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  (the abstract constructor) is injective*}
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theorem exprel_imp_eq_freevars: "U \<sim> V \<Longrightarrow> freevars U = freevars V"
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apply (induct set: exprel) 
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apply (erule_tac [4] listrel.induct) 
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apply (simp_all add: Un_assoc)
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done
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subsubsection{*Functions for Freeness*}
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text{*A discriminator function to distinguish vars, sums and function calls*}
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primrec freediscrim :: "freeExp \<Rightarrow> int" where
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  "freediscrim (VAR N) = 0"
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| "freediscrim (PLUS X Y) = 1"
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| "freediscrim (FNCALL F Xs) = 2"
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theorem exprel_imp_eq_freediscrim:
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     "U \<sim> V \<Longrightarrow> freediscrim U = freediscrim V"
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  by (induct set: exprel) auto
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text{*This function, which returns the function name, is used to
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prove part of the injectivity property for FnCall.*}
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primrec freefun :: "freeExp \<Rightarrow> nat" where
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  "freefun (VAR N) = 0"
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| "freefun (PLUS X Y) = 0"
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| "freefun (FNCALL F Xs) = F"
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theorem exprel_imp_eq_freefun:
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     "U \<sim> V \<Longrightarrow> freefun U = freefun V"
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  by (induct set: exprel) (simp_all add: listrel.intros)
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text{*This function, which returns the list of function arguments, is used to
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prove part of the injectivity property for FnCall.*}
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primrec freeargs :: "freeExp \<Rightarrow> freeExp list" where
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  "freeargs (VAR N) = []"
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| "freeargs (PLUS X Y) = []"
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| "freeargs (FNCALL F Xs) = Xs"
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theorem exprel_imp_eqv_freeargs:
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     "U \<sim> V \<Longrightarrow> (freeargs U, freeargs V) \<in> listrel exprel"
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apply (induct set: exprel)
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apply (erule_tac [4] listrel.induct) 
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apply (simp_all add: listrel.intros)
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apply (blast intro: symD [OF equiv.sym [OF equiv_list_exprel]])
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apply (blast intro: transD [OF equiv.trans [OF equiv_list_exprel]])
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done
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subsection{*The Initial Algebra: A Quotiented Message Type*}
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typedef (Exp)  exp = "UNIV//exprel"
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    by (auto simp add: quotient_def)
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text{*The abstract message constructors*}
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definition
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  Var :: "nat \<Rightarrow> exp" where
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  "Var N = Abs_Exp(exprel``{VAR N})"
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definition
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  Plus :: "[exp,exp] \<Rightarrow> exp" where
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   "Plus X Y =
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       Abs_Exp (\<Union>U \<in> Rep_Exp X. \<Union>V \<in> Rep_Exp Y. exprel``{PLUS U V})"
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definition
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  FnCall :: "[nat, exp list] \<Rightarrow> exp" where
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   "FnCall F Xs =
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       Abs_Exp (\<Union>Us \<in> listset (map Rep_Exp Xs). exprel `` {FNCALL F Us})"
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text{*Reduces equality of equivalence classes to the @{term exprel} relation:
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  @{term "(exprel `` {x} = exprel `` {y}) = ((x,y) \<in> exprel)"} *}
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lemmas equiv_exprel_iff = eq_equiv_class_iff [OF equiv_exprel UNIV_I UNIV_I]
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declare equiv_exprel_iff [simp]
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text{*All equivalence classes belong to set of representatives*}
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lemma [simp]: "exprel``{U} \<in> Exp"
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by (auto simp add: Exp_def quotient_def intro: exprel_refl)
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lemma inj_on_Abs_Exp: "inj_on Abs_Exp Exp"
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apply (rule inj_on_inverseI)
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apply (erule Abs_Exp_inverse)
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done
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text{*Reduces equality on abstractions to equality on representatives*}
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declare inj_on_Abs_Exp [THEN inj_on_iff, simp]
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declare Abs_Exp_inverse [simp]
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text{*Case analysis on the representation of a exp as an equivalence class.*}
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lemma eq_Abs_Exp [case_names Abs_Exp, cases type: exp]:
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     "(!!U. z = Abs_Exp(exprel``{U}) ==> P) ==> P"
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apply (rule Rep_Exp [of z, unfolded Exp_def, THEN quotientE])
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apply (drule arg_cong [where f=Abs_Exp])
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apply (auto simp add: Rep_Exp_inverse intro: exprel_refl)
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done
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subsection{*Every list of abstract expressions can be expressed in terms of a
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  list of concrete expressions*}
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definition
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  Abs_ExpList :: "freeExp list => exp list" where
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  "Abs_ExpList Xs = map (%U. Abs_Exp(exprel``{U})) Xs"
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lemma Abs_ExpList_Nil [simp]: "Abs_ExpList [] == []"
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by (simp add: Abs_ExpList_def)
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lemma Abs_ExpList_Cons [simp]:
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     "Abs_ExpList (X#Xs) == Abs_Exp (exprel``{X}) # Abs_ExpList Xs"
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by (simp add: Abs_ExpList_def)
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lemma ExpList_rep: "\<exists>Us. z = Abs_ExpList Us"
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apply (induct z)
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apply (rule_tac [2] z=a in eq_Abs_Exp)
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apply (auto simp add: Abs_ExpList_def Cons_eq_map_conv intro: exprel_refl)
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done
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lemma eq_Abs_ExpList [case_names Abs_ExpList]:
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     "(!!Us. z = Abs_ExpList Us ==> P) ==> P"
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by (rule exE [OF ExpList_rep], blast) 
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subsubsection{*Characteristic Equations for the Abstract Constructors*}
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lemma Plus: "Plus (Abs_Exp(exprel``{U})) (Abs_Exp(exprel``{V})) = 
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             Abs_Exp (exprel``{PLUS U V})"
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proof -
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  have "(\<lambda>U V. exprel `` {PLUS U V}) respects2 exprel"
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    by (simp add: congruent2_def exprel.PLUS)
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  thus ?thesis
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    by (simp add: Plus_def UN_equiv_class2 [OF equiv_exprel equiv_exprel])
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qed
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text{*It is not clear what to do with FnCall: it's argument is an abstraction
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of an @{typ "exp list"}. Is it just Nil or Cons? What seems to work best is to
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regard an @{typ "exp list"} as a @{term "listrel exprel"} equivalence class*}
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text{*This theorem is easily proved but never used. There's no obvious way
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even to state the analogous result, @{text FnCall_Cons}.*}
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lemma FnCall_Nil: "FnCall F [] = Abs_Exp (exprel``{FNCALL F []})"
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  by (simp add: FnCall_def)
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lemma FnCall_respects: 
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     "(\<lambda>Us. exprel `` {FNCALL F Us}) respects (listrel exprel)"
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  by (simp add: congruent_def exprel.FNCALL)
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lemma FnCall_sing:
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     "FnCall F [Abs_Exp(exprel``{U})] = Abs_Exp (exprel``{FNCALL F [U]})"
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proof -
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  have "(\<lambda>U. exprel `` {FNCALL F [U]}) respects exprel"
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    by (simp add: congruent_def FNCALL_Cons listrel.intros)
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  thus ?thesis
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    by (simp add: FnCall_def UN_equiv_class [OF equiv_exprel])
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qed
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lemma listset_Rep_Exp_Abs_Exp:
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     "listset (map Rep_Exp (Abs_ExpList Us)) = listrel exprel `` {Us}";
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  by (induct Us) (simp_all add: listrel_Cons Abs_ExpList_def)
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lemma FnCall:
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     "FnCall F (Abs_ExpList Us) = Abs_Exp (exprel``{FNCALL F Us})"
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proof -
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  have "(\<lambda>Us. exprel `` {FNCALL F Us}) respects (listrel exprel)"
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    by (simp add: congruent_def exprel.FNCALL)
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  thus ?thesis
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    by (simp add: FnCall_def UN_equiv_class [OF equiv_list_exprel]
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                  listset_Rep_Exp_Abs_Exp)
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qed
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text{*Establishing this equation is the point of the whole exercise*}
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theorem Plus_assoc: "Plus X (Plus Y Z) = Plus (Plus X Y) Z"
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by (cases X, cases Y, cases Z, simp add: Plus exprel.ASSOC)
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subsection{*The Abstract Function to Return the Set of Variables*}
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definition
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  vars :: "exp \<Rightarrow> nat set" where
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  "vars X = (\<Union>U \<in> Rep_Exp X. freevars U)"
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lemma vars_respects: "freevars respects exprel"
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by (simp add: congruent_def exprel_imp_eq_freevars) 
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text{*The extension of the function @{term vars} to lists*}
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primrec vars_list :: "exp list \<Rightarrow> nat set" where
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  "vars_list []    = {}"
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| "vars_list(E#Es) = vars E \<union> vars_list Es"
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text{*Now prove the three equations for @{term vars}*}
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lemma vars_Variable [simp]: "vars (Var N) = {N}"
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by (simp add: vars_def Var_def 
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              UN_equiv_class [OF equiv_exprel vars_respects]) 
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lemma vars_Plus [simp]: "vars (Plus X Y) = vars X \<union> vars Y"
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apply (cases X, cases Y) 
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apply (simp add: vars_def Plus 
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                 UN_equiv_class [OF equiv_exprel vars_respects]) 
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done
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lemma vars_FnCall [simp]: "vars (FnCall F Xs) = vars_list Xs"
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apply (cases Xs rule: eq_Abs_ExpList) 
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apply (simp add: FnCall)
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apply (induct_tac Us) 
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apply (simp_all add: vars_def UN_equiv_class [OF equiv_exprel vars_respects])
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done
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lemma vars_FnCall_Nil: "vars (FnCall F Nil) = {}" 
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by simp
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lemma vars_FnCall_Cons: "vars (FnCall F (X#Xs)) = vars X \<union> vars_list Xs"
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by simp
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subsection{*Injectivity Properties of Some Constructors*}
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lemma VAR_imp_eq: "VAR m \<sim> VAR n \<Longrightarrow> m = n"
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by (drule exprel_imp_eq_freevars, simp)
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text{*Can also be proved using the function @{term vars}*}
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lemma Var_Var_eq [iff]: "(Var m = Var n) = (m = n)"
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by (auto simp add: Var_def exprel_refl dest: VAR_imp_eq)
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   320
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lemma VAR_neqv_PLUS: "VAR m \<sim> PLUS X Y \<Longrightarrow> False"
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by (drule exprel_imp_eq_freediscrim, simp)
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theorem Var_neq_Plus [iff]: "Var N \<noteq> Plus X Y"
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apply (cases X, cases Y) 
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apply (simp add: Var_def Plus) 
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apply (blast dest: VAR_neqv_PLUS) 
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done
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   329
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theorem Var_neq_FnCall [iff]: "Var N \<noteq> FnCall F Xs"
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apply (cases Xs rule: eq_Abs_ExpList) 
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apply (auto simp add: FnCall Var_def)
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apply (drule exprel_imp_eq_freediscrim, simp)
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done
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subsection{*Injectivity of @{term FnCall}*}
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definition
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  "fun" :: "exp \<Rightarrow> nat" where
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  "fun X = contents (\<Union>U \<in> Rep_Exp X. {freefun U})"
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lemma fun_respects: "(%U. {freefun U}) respects exprel"
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by (simp add: congruent_def exprel_imp_eq_freefun) 
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lemma fun_FnCall [simp]: "fun (FnCall F Xs) = F"
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apply (cases Xs rule: eq_Abs_ExpList) 
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apply (simp add: FnCall fun_def UN_equiv_class [OF equiv_exprel fun_respects])
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done
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definition
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  args :: "exp \<Rightarrow> exp list" where
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  "args X = contents (\<Union>U \<in> Rep_Exp X. {Abs_ExpList (freeargs U)})"
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text{*This result can probably be generalized to arbitrary equivalence
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relations, but with little benefit here.*}
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lemma Abs_ExpList_eq:
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     "(y, z) \<in> listrel exprel \<Longrightarrow> Abs_ExpList (y) = Abs_ExpList (z)"
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  by (induct set: listrel) simp_all
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lemma args_respects: "(%U. {Abs_ExpList (freeargs U)}) respects exprel"
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by (simp add: congruent_def Abs_ExpList_eq exprel_imp_eqv_freeargs) 
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   362
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lemma args_FnCall [simp]: "args (FnCall F Xs) = Xs"
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apply (cases Xs rule: eq_Abs_ExpList) 
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apply (simp add: FnCall args_def UN_equiv_class [OF equiv_exprel args_respects])
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done
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   367
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lemma FnCall_FnCall_eq [iff]:
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     "(FnCall F Xs = FnCall F' Xs') = (F=F' & Xs=Xs')" 
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proof
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  assume "FnCall F Xs = FnCall F' Xs'"
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  hence "fun (FnCall F Xs) = fun (FnCall F' Xs')" 
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    and "args (FnCall F Xs) = args (FnCall F' Xs')" by auto
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  thus "F=F' & Xs=Xs'" by simp
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next
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  assume "F=F' & Xs=Xs'" thus "FnCall F Xs = FnCall F' Xs'" by simp
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qed
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   379
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subsection{*The Abstract Discriminator*}
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text{*However, as @{text FnCall_Var_neq_Var} illustrates, we don't need this
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function in order to prove discrimination theorems.*}
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definition
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  discrim :: "exp \<Rightarrow> int" where
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  "discrim X = contents (\<Union>U \<in> Rep_Exp X. {freediscrim U})"
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lemma discrim_respects: "(\<lambda>U. {freediscrim U}) respects exprel"
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by (simp add: congruent_def exprel_imp_eq_freediscrim) 
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   391
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text{*Now prove the four equations for @{term discrim}*}
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   393
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lemma discrim_Var [simp]: "discrim (Var N) = 0"
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by (simp add: discrim_def Var_def 
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              UN_equiv_class [OF equiv_exprel discrim_respects]) 
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   397
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lemma discrim_Plus [simp]: "discrim (Plus X Y) = 1"
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apply (cases X, cases Y) 
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apply (simp add: discrim_def Plus 
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                 UN_equiv_class [OF equiv_exprel discrim_respects]) 
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done
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parents:
diff changeset
   403
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   404
lemma discrim_FnCall [simp]: "discrim (FnCall F Xs) = 2"
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   405
apply (rule_tac z=Xs in eq_Abs_ExpList) 
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   406
apply (simp add: discrim_def FnCall
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   407
                 UN_equiv_class [OF equiv_exprel discrim_respects]) 
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   408
done
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diff changeset
   409
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diff changeset
   410
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text{*The structural induction rule for the abstract type*}
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theorem exp_inducts:
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  assumes V:    "\<And>nat. P1 (Var nat)"
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      and P:    "\<And>exp1 exp2. \<lbrakk>P1 exp1; P1 exp2\<rbrakk> \<Longrightarrow> P1 (Plus exp1 exp2)"
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      and F:    "\<And>nat list. P2 list \<Longrightarrow> P1 (FnCall nat list)"
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   416
      and Nil:  "P2 []"
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   417
      and Cons: "\<And>exp list. \<lbrakk>P1 exp; P2 list\<rbrakk> \<Longrightarrow> P2 (exp # list)"
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   418
  shows "P1 exp" and "P2 list"
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   419
proof -
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   420
  obtain U where exp: "exp = (Abs_Exp (exprel `` {U}))" by (cases exp)
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   421
  obtain Us where list: "list = Abs_ExpList Us" by (rule eq_Abs_ExpList)
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   422
  have "P1 (Abs_Exp (exprel `` {U}))" and "P2 (Abs_ExpList Us)"
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  proof (induct U and Us)
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   424
    case (VAR nat)
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    with V show ?case by (simp add: Var_def) 
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   426
  next
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   427
    case (PLUS X Y)
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   428
    with P [of "Abs_Exp (exprel `` {X})" "Abs_Exp (exprel `` {Y})"]
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parents:
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   429
    show ?case by (simp add: Plus) 
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   430
  next
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   431
    case (FNCALL nat list)
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   432
    with F [of "Abs_ExpList list"]
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paulson
parents:
diff changeset
   433
    show ?case by (simp add: FnCall) 
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parents:
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   434
  next
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   435
    case Nil_freeExp
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   436
    with Nil show ?case by simp
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parents:
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   437
  next
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parents:
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   438
    case Cons_freeExp
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parents: 18447
diff changeset
   439
    with Cons show ?case by simp
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parents:
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   440
  qed
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parents: 18447
diff changeset
   441
  with exp and list show "P1 exp" and "P2 list" by (simp_all only:)
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paulson
parents:
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   442
qed
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parents:
diff changeset
   443
73069e033a0b new example of a quotiented nested data type
paulson
parents:
diff changeset
   444
end