theory Ind_Examples+
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imports Main LaTeXsugar+
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begin+
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+
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section{* Examples of Inductive Definitions \label{sec:ind-examples} *}+
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+
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text {*+
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+
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  In this section, we will give three examples showing how to define inductive+
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  predicates by hand and prove characteristic properties such as introduction+
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  rules and an induction rule. From these examples, we will then figure out a+
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  general method for defining inductive predicates.  It should be noted that+
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  our aim in this section is not to write proofs that are as beautiful as+
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  possible, but as close as possible to the ML code producing the proofs that+
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  we will develop later.  As a first example, we consider the transitive+
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  closure of a relation @{text R}. It is an inductive predicate characterized+
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  by the two introduction rules+
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+
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  \begin{center}+
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  @{term[mode=Axiom] "trcl R x x"} \hspace{5mm}+
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  @{term[mode=Rule] "R x y \<Longrightarrow> trcl R y z \<Longrightarrow> trcl R x z"}+
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  \end{center}+
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+
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  (FIXME first rule should be an ``axiom'')+
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+
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  Note that the @{text trcl} predicate has two different kinds of parameters: the+
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  first parameter @{text R} stays \emph{fixed} throughout the definition, whereas+
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  the second and third parameter changes in the ``recursive call''.+
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+
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  Since an inductively defined predicate is the least predicate closed under+
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  a collection of introduction rules, we define the predicate @{text "trcl R x y"} in+
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  such a way that it holds if and only if @{text "P x y"} holds for every predicate+
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  @{text P} closed under the rules above. This gives rise to the definition +
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*}+
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+
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definition "trcl R x y \<equiv> +
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      \<forall>P. (\<forall>x. P x x) \<longrightarrow> (\<forall>x y z. R x y \<longrightarrow> P y z \<longrightarrow> P x z) \<longrightarrow> P x y"+
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+
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text {*+
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  where we quantify over the predicate @{text P}.+
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+
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  Since the predicate @{term "trcl R x y"} yields an element of the type of object+
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  level truth values @{text bool}, the meta-level implications @{text "\<Longrightarrow>"} in the+
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  above introduction rules have to be converted to object-level implications+
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  @{text "\<longrightarrow>"}. Moreover, we use object-level universal quantifiers @{text "\<forall>"}+
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  rather than meta-level universal quantifiers @{text "\<And>"} for quantifying over+
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  the variable parameters of the introduction rules. Isabelle already offers some+
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  infrastructure for converting between meta-level and object-level connectives,+
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  which we will use later on.+
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+
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  With this definition of the transitive closure, the proof of the (weak) induction+
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  theorem is almost immediate. It suffices to convert all the meta-level connectives+
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  in the induction rule to object-level connectives using the @{text atomize} proof+
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  method, expand the definition of @{text trcl}, eliminate the universal quantifier+
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  contained in it, and then solve the goal by assumption.+
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*}+
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+
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lemma trcl_induct:+
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  assumes trcl: "trcl R x y"+
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  shows "(\<And>x. P x x) \<Longrightarrow> (\<And>x y z. R x y \<Longrightarrow> P y z \<Longrightarrow> P x z) \<Longrightarrow> P x y"+
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  apply (atomize (full))+
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  apply (cut_tac trcl)+
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  apply (unfold trcl_def)+
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  apply (drule spec [where x=P])+
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  apply assumption+
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  done+
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(*<*)+
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lemma "trcl R x x"+
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  apply (unfold trcl_def)+
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  apply (rule allI impI)++
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(*>*)+
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+
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txt {*+
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+
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  The above induction rule is \emph{weak} in the sense that the induction step may+
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  only be proved using the assumptions @{term "R x y"} and @{term "P y z"}, but not+
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  using the additional assumption \mbox{@{term "trcl R y z"}}. A stronger induction rule+
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  containing this additional assumption can be derived from the weaker one with the+
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  help of the introduction rules for @{text trcl}.+
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+
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  We now turn to the proofs of the introduction rules, which are slightly more complicated.+
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  In order to prove the first introduction rule, we again unfold the definition and+
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  then apply the introdution rules for @{text "\<forall>"} and @{text "\<longrightarrow>"} as often as possible.+
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  We then end up in a proof state of the following form:+
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  +
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  @{subgoals [display]}+
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+
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  The two assumptions correspond to the introduction rules, where @{term "trcl R"} has been+
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  replaced by @{term "P"}. Thus, all we have to do is to eliminate the universal quantifier+
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  in front of the first assumption, and then solve the goal by assumption:+
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  *}+
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(*<*)oops(*>*)+
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+
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lemma trcl_base: "trcl R x x"+
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  apply (unfold trcl_def)+
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  apply (rule allI impI)++
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  apply (drule spec)+
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  apply assumption+
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  done+
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+
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(*<*)+
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lemma "R x y \<Longrightarrow> trcl R y z \<Longrightarrow> trcl R x z"+
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  apply (unfold trcl_def)+
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  apply (rule allI impI)++
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(*>*)+
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+
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txt {*+
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+
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  Since the second introduction rule has premises, its proof is not as easy as the previous+
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  one. After unfolding the definitions and applying the introduction rules for @{text "\<forall>"}+
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  and @{text "\<longrightarrow>"}, we get the proof state+
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  @{subgoals [display]}+
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  The third and fourth assumption corresponds to the first and second introduction rule,+
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  respectively, whereas the first and second assumption corresponds to the premises of+
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  the introduction rule. Since we want to prove the second introduction rule, we apply+
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  the fourth assumption to the goal @{term "P x z"}. In order for the assumption to+
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  be applicable, we have to eliminate the universal quantifiers and turn the object-level+
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  implications into meta-level ones. This can be accomplished using the @{text rule_format}+
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  attribute. Applying the assumption produces two new subgoals, which can be solved using+
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  the first and second assumption. The second assumption again involves a quantifier and+
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  implications that have to be eliminated before it can be applied. To avoid problems+
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  with higher order unification, it is advisable to provide an instantiation for the+
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  universally quantified predicate variable in the assumption.+
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*}+
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(*<*)oops(*>*)+
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+
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lemma trcl_step: "R x y \<Longrightarrow> trcl R y z \<Longrightarrow> trcl R x z"+
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  apply (unfold trcl_def)+
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  apply (rule allI impI)++
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  proof -+
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    case goal1+
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    show ?case+
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      apply (rule goal1(4) [rule_format])+
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      apply (rule goal1(1))+
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      apply (rule goal1(2) [THEN spec [where x=P], THEN mp, THEN mp,+
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	OF goal1(3-4)])+
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      done+
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  qed+
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+
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text {*+
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+
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  This method of defining inductive predicates easily generalizes to mutually inductive+
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  predicates, like the predicates @{text even} and @{text odd} characterized by the+
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  following introduction rules:+
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 +
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  \begin{center}+
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  @{term[mode=Axiom] "even (0::nat)"} \hspace{5mm}+
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  @{term[mode=Rule] "odd m \<Longrightarrow> even (Suc m)"} \hspace{5mm}+
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  @{term[mode=Rule] "even m \<Longrightarrow> odd (Suc m)"}+
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  \end{center}+
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  +
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  Since the predicates are mutually inductive, each of the definitions contain two+
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  quantifiers over the predicates @{text P} and @{text Q}.+
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*}+
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+
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definition "even n \<equiv> +
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  \<forall>P Q. P 0 \<longrightarrow> (\<forall>m. Q m \<longrightarrow> P (Suc m)) +
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             \<longrightarrow> (\<forall>m. P m \<longrightarrow> Q (Suc m)) \<longrightarrow> P n"+
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+
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definition "odd n \<equiv>+
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  \<forall>P Q. P 0 \<longrightarrow> (\<forall>m. Q m \<longrightarrow> P (Suc m)) +
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             \<longrightarrow> (\<forall>m. P m \<longrightarrow> Q (Suc m)) \<longrightarrow> Q n"+
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+
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text {*+
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  For proving the induction rule, we use exactly the same technique as in the transitive+
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  closure example:+
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*}+
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+
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lemma even_induct:+
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  assumes even: "even n"+
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  shows "P 0 \<Longrightarrow> +
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             (\<And>m. Q m \<Longrightarrow> P (Suc m)) \<Longrightarrow> (\<And>m. P m \<Longrightarrow> Q (Suc m)) \<Longrightarrow> P n"+
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  apply (atomize (full))+
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  apply (cut_tac even)+
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  apply (unfold even_def)+
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  apply (drule spec [where x=P])+
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  apply (drule spec [where x=Q])+
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  apply assumption+
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  done+
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+
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text {*+
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  A similar induction rule having @{term "Q n"} as a conclusion can be proved for+
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  the @{text odd} predicate.+
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  The proofs of the introduction rules are also very similar to the ones in the+
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  previous example. We only show the proof of the second introduction rule,+
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  since it is almost the same as the one for the third introduction rule,+
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  and the proof of the first rule is trivial.+
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*}+
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+
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lemma evenS: "odd m \<Longrightarrow> even (Suc m)"+
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  apply (unfold odd_def even_def)+
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  apply (rule allI impI)++
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  proof -+
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    case goal1+
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    show ?case+
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      apply (rule goal1(3) [rule_format])+
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      apply (rule goal1(1) [THEN spec [where x=P], THEN spec [where x=Q],+
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	THEN mp, THEN mp, THEN mp, OF goal1(2-4)])+
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      done+
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  qed+
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(*<*)+
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lemma even0: "even 0"+
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  apply (unfold even_def)+
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  apply (rule allI impI)++
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  apply assumption+
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  done+
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+
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lemma oddS: "even m \<Longrightarrow> odd (Suc m)"+
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  apply (unfold odd_def even_def)+
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  apply (rule allI impI)++
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  proof -+
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    case goal1+
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    show ?case+
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      apply (rule goal1(4) [rule_format])+
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      apply (rule goal1(1) [THEN spec [where x=P], THEN spec [where x=Q],+
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	THEN mp, THEN mp, THEN mp, OF goal1(2-4)])+
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      done+
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  qed+
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(*>*)+
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+
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text {*+
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  As a final example, we will consider the definition of the accessible+
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  part of a relation @{text R} characterized by the introduction rule+
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  +
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  \begin{center}+
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  @{term[mode=Rule] "(\<forall>y. R y x \<longrightarrow> accpart R y) \<Longrightarrow> accpart R x"}+
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  \end{center}+
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+
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  whose premise involves a universal quantifier and an implication. The+
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  definition of @{text accpart} is as follows:+
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*}+
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+
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definition "accpart R x \<equiv> \<forall>P. (\<forall>x. (\<forall>y. R y x \<longrightarrow> P y) \<longrightarrow> P x) \<longrightarrow> P x"+
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+
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text {*+
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  The proof of the induction theorem is again straightforward:+
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*}+
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+
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lemma accpart_induct:+
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  assumes acc: "accpart R x"+
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  shows "(\<And>x. (\<And>y. R y x \<Longrightarrow> P y) \<Longrightarrow> P x) \<Longrightarrow> P x"+
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  apply (atomize (full))+
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  apply (cut_tac acc)+
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  apply (unfold accpart_def)+
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  apply (drule spec [where x=P])+
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  apply assumption+
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  done+
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(*<*)+
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lemma accpartI: "(\<And>y. R y x \<Longrightarrow> accpart R y) \<Longrightarrow> accpart R x"+
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  apply (unfold accpart_def)+
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  apply (rule allI impI)++
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(*>*)+
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+
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txt {*+
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+
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  Proving the introduction rule is a little more complicated, due to the quantifier+
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  and the implication in the premise. We first convert the meta-level universal quantifier+
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  and implication to their object-level counterparts. Unfolding the definition of+
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  @{text accpart} and applying the introduction rules for @{text "\<forall>"} and @{text "\<longrightarrow>"}+
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  yields the following proof state:+
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  @{subgoals [display]}+
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  Applying the second assumption produces a proof state with the new local assumption+
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  @{term "R y x"}, which will then be used to solve the goal @{term "P y"} using the+
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  first assumption.+
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*}+
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(*<*)oops(*>*)+
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+
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lemma accpartI: "(\<And>y. R y x \<Longrightarrow> accpart R y) \<Longrightarrow> accpart R x"+
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  apply (unfold accpart_def)+
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  apply (rule allI impI)++
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  proof -+
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    case goal1+
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    note goal1' = this+
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    show ?case+
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      apply (rule goal1'(2) [rule_format])+
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      proof -+
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        case goal1+
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        show ?case+
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	  apply (rule goal1'(1) [OF goal1, THEN spec [where x=P],+
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            THEN mp, OF goal1'(2)])+
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	  done+
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      qed+
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    qed+
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+
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end+
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