theory Ind_Prelims

imports Main "../Base" 

begin

section{* Preliminaries *}

text {*

  The user will just give a specification of inductive predicate(s) and

  expects from the package to produce a convenient reasoning

  infrastructure. This infrastructure needs to be derived from the definition

  that correspond to the specified predicate(s). Before we start with

  explaining all parts of the package, let us first give some examples 

  showing how to define inductive predicates and then also how

  to generate a reasoning infrastructure for them. From the examples 

  we will figure out a general method for

  defining inductive predicates.  The aim in this section is \emph{not} to

  write proofs that are as beautiful as possible, but as close as possible to

  the ML-implementation we will develop in later sections.

  We first consider the transitive closure of a relation @{text R}. The 

  ``pencil-and-paper'' specification for the transitive closure is:

  \begin{center}\small

  @{prop[mode=Axiom] "trcl R x x"} \hspace{5mm}

  @{prop[mode=Rule] "R x y \<Longrightarrow> trcl R y z \<Longrightarrow> trcl R x z"}

  \end{center}

  The package has to make an appropriate definition for @{term "trcl"}. 

  Since an inductively

  defined predicate is the least predicate closed under a collection of

  introduction rules, the predicate @{text "trcl R x y"} can be defined so

  that it holds if and only if @{text "P x y"} holds for every predicate

  @{text P} closed under the rules above. This gives rise to the definition

*}

definition "trcl \<equiv> 

     \<lambda>R x y. \<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"

text {*

  We have to use the object implication @{text "\<longrightarrow>"} and object quantification

  @{text "\<forall>"} for stating this definition (there is no other way for

  definitions in HOL). However, the introduction rules and induction

  principles associated with the transitive closure should use the meta-connectives, 

  since they simplify the reasoning for the user.

  With this definition, the proof of the induction principle for @{term trcl}

  is almost immediate. It suffices to convert all the meta-level

  connectives in the lemma to object-level connectives using the

  proof method @{text atomize} (Line 4 below), expand the definition of @{term trcl}

  (Line 5 and 6), eliminate the universal quantifier contained in it (Line~7),

  and then solve the goal by @{text assumption} (Line 8).

*}

lemma %linenos trcl_induct:

assumes "trcl R x y"

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"

apply(atomize (full))

apply(cut_tac prems)

apply(unfold trcl_def)

apply(drule spec[where x=P])

apply(assumption)

done

text {*

  The proofs for the introduction rules are slightly more complicated. 

  For the first one, we need to prove the following lemma:

*}

lemma %linenos trcl_base: 

shows "trcl R x x"

apply(unfold trcl_def)

apply(rule allI impI)+

apply(drule spec)

apply(assumption)

done

text {*

  We again unfold first the definition and apply introduction rules 

  for @{text "\<forall>"} and @{text "\<longrightarrow>"} as often as possible (Lines 3 and 4).

  We then end up in the goal state:

*}

(*<*)lemma "trcl R x x"

apply (unfold trcl_def)

apply (rule allI impI)+(*>*)

txt {* @{subgoals [display]} *}

(*<*)oops(*>*)

text {*

  The two assumptions come from the definition of @{term trcl} and correspond

  to the introduction rules. Thus, all we have to do is to eliminate the

  universal quantifier in front of the first assumption (Line 5), and then

  solve the goal by @{text assumption} (Line 6).

*}

text {*

  Next we have to show that the second introduction rule also follows from the

  definition.  Since this rule has premises, the proof is a bit more

  involved. After unfolding the definitions and applying the introduction

  rules for @{text "\<forall>"} and @{text "\<longrightarrow>"}

*}

lemma trcl_step: 

shows "R x y \<Longrightarrow> trcl R y z \<Longrightarrow> trcl R x z"

apply (unfold trcl_def)

apply (rule allI impI)+

txt {* 

  we obtain the goal state

  @{subgoals [display]} 

  To see better where we are, let us explicitly name the assumptions 

  by starting a subproof.

*}

proof -

  case (goal1 P)

  have p1: "R x y" by fact

  have p2: "\<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 y z" by fact

  have r1: "\<forall>x. P x x" by fact

  have r2: "\<forall>x y z. R x y \<longrightarrow> P y z \<longrightarrow> P x z" by fact

  show "P x z"

txt {*

  The assumptions @{text "p1"} and @{text "p2"} correspond to the premises of

  the second introduction rule (unfolded); the assumptions @{text "r1"} and @{text "r2"}

  come from the definition of @{term trcl}. We apply @{text "r2"} to the goal

  @{term "P x z"}. In order for this assumption to be applicable as a rule, we

  have to eliminate the universal quantifier and turn the object-level

  implications into meta-level ones. This can be accomplished using the @{text

  rule_format} attribute. So we continue the proof with:

*}

    apply (rule r2[rule_format])

txt {*

  This gives us two new subgoals

  @{subgoals [display]} 

  which can be solved using assumptions @{text p1} and @{text p2}. The latter

  involves a quantifier and implications that have to be eliminated before it

  can be applied. To avoid potential problems with higher-order unification,

  we explicitly instantiate the quantifier to @{text "P"} and also match

  explicitly the implications with @{text "r1"} and @{text "r2"}. This gives

  the proof:

*}

    apply(rule p1)

    apply(rule p2[THEN spec[where x=P], THEN mp, THEN mp, OF r1, OF r2])

    done

qed

text {*

  Now we are done. It might be surprising that we are not using the automatic

  tactics available in Isabelle/HOL for proving this lemmas. After all @{text

  "blast"} would easily dispense of it.

*}

lemma trcl_step_blast: 

shows "R x y \<Longrightarrow> trcl R y z \<Longrightarrow> trcl R x z"

apply(unfold trcl_def)

apply(blast)

done

text {*

  Experience has shown that it is generally a bad idea to rely heavily on

  @{text blast}, @{text auto} and the like in automated proofs. The reason is

  that you do not have precise control over them (the user can, for example,

  declare new intro- or simplification rules that can throw automatic tactics

  off course) and also it is very hard to debug proofs involving automatic

  tactics whenever something goes wrong. Therefore if possible, automatic 

  tactics should be avoided or be constrained sufficiently.

  The method of defining inductive predicates by impredicative quantification

  also generalises to mutually inductive predicates. The next example defines

  the predicates @{text even} and @{text odd} given by

  \begin{center}\small

  @{prop[mode=Axiom] "even (0::nat)"} \hspace{5mm}

  @{prop[mode=Rule] "odd n \<Longrightarrow> even (Suc n)"} \hspace{5mm}

  @{prop[mode=Rule] "even n \<Longrightarrow> odd (Suc n)"}

  \end{center}

  Since the predicates @{term even} and @{term odd} are mutually inductive, each 

  corresponding definition must quantify over both predicates (we name them 

  below @{text "P"} and @{text "Q"}).

*}

definition "even \<equiv> 

  \<lambda>n. \<forall>P Q. P 0 \<longrightarrow> (\<forall>m. Q m \<longrightarrow> P (Suc m)) 

                 \<longrightarrow> (\<forall>m. P m \<longrightarrow> Q (Suc m)) \<longrightarrow> P n"

definition "odd \<equiv>

  \<lambda>n. \<forall>P Q. P 0 \<longrightarrow> (\<forall>m. Q m \<longrightarrow> P (Suc m)) 

                 \<longrightarrow> (\<forall>m. P m \<longrightarrow> Q (Suc m)) \<longrightarrow> Q n"

text {*

  For proving the induction principles, we use exactly the same technique 

  as in the transitive closure example, namely:

*}

lemma even_induct:

assumes "even n"

shows "P 0 \<Longrightarrow> (\<And>m. Q m \<Longrightarrow> P (Suc m)) \<Longrightarrow> (\<And>m. P m \<Longrightarrow> Q (Suc m)) \<Longrightarrow> P n"

apply(atomize (full))

apply(cut_tac prems)

apply(unfold even_def)

apply(drule spec[where x=P])

apply(drule spec[where x=Q])

apply(assumption)

done

text {*

  The only difference with the proof @{text "trcl_induct"} is that we have to

  instantiate here two universal quantifiers.  We omit the other induction

  principle that has @{prop "even n"} as premise and @{term "Q n"} as conclusion.  

  The proofs of the introduction rules are also very similar to the ones in 

  the @{text "trcl"}-example. We only show the proof of the second introduction 

  rule.

*}

lemma %linenos evenS: 

shows "odd m \<Longrightarrow> even (Suc m)"

apply (unfold odd_def even_def)

apply (rule allI impI)+

proof -

  case (goal1 P Q)

  have p1: "\<forall>P Q. P 0 \<longrightarrow> (\<forall>m. Q m \<longrightarrow> P (Suc m)) 

                             \<longrightarrow> (\<forall>m. P m \<longrightarrow> Q (Suc m)) \<longrightarrow> Q m" by fact

  have r1: "P 0" by fact

  have r2: "\<forall>m. Q m \<longrightarrow> P (Suc m)" by fact

  have r3: "\<forall>m. P m \<longrightarrow> Q (Suc m)" by fact

  show "P (Suc m)"

    apply(rule r2[rule_format])

    apply(rule p1[THEN spec[where x=P], THEN spec[where x=Q],

	           THEN mp, THEN mp, THEN mp, OF r1, OF r2, OF r3])

    done

qed

text {*

  The interesting lines are 7 to 15. The assumptions fall into two categories:

  @{text p1} corresponds to the premise of the introduction rule; @{text "r1"}

  to @{text "r3"} come from the definition of @{text "even"}.

  In Line 13, we apply the assumption @{text "r2"} (since we prove the second

  introduction rule). In Lines 14 and 15 we apply assumption @{text "p1"} (if

  the second introduction rule had more premises we have to do that for all

  of them). In order for this assumption to be applicable, the quantifiers

  need to be instantiated and then also the implications need to be resolved

  with the other rules.

  Next we define the accessible part of a relation @{text R} given by

  the single rule:

  \begin{center}\small

  \mbox{\inferrule{@{term "\<And>y. R y x \<Longrightarrow> accpart R y"}}{@{term "accpart R x"}}}

  \end{center}

  The definition of @{text "accpart"} is:

*}

definition "accpart \<equiv> \<lambda>R x. \<forall>P. (\<forall>x. (\<forall>y. R y x \<longrightarrow> P y) \<longrightarrow> P x) \<longrightarrow> P x"

text {*

  The proof of the induction principle is again straightforward and omitted.

  Proving the introduction rule is a little more complicated, because the 

  quantifier and the implication in the premise. The proof is as follows.

*}

lemma %linenos accpartI: 

shows "(\<And>y. R y x \<Longrightarrow> accpart R y) \<Longrightarrow> accpart R x"

apply (unfold accpart_def)

apply (rule allI impI)+

proof -

  case (goal1 P)

  have p1: "\<And>y. R y x \<Longrightarrow> 

                   (\<forall>P. (\<forall>x. (\<forall>y. R y x \<longrightarrow> P y) \<longrightarrow> P x) \<longrightarrow> P y)" by fact

  have r1: "\<forall>x. (\<forall>y. R y x \<longrightarrow> P y) \<longrightarrow> P x" by fact

  show "P x"

    apply(rule r1[rule_format])

    proof -

      case (goal1 y)

      have r1_prem: "R y x" by fact

      show "P y"

	apply(rule p1[OF r1_prem, THEN spec[where x=P], THEN mp, OF r1])

      done

  qed

qed

text {*

  As you can see, there are now two subproofs. The assumptions fall again into

  two categories (Lines 7 to 9). In Line 11, applying the assumption @{text

  "r1"} generates a goal state with the new local assumption @{term "R y x"},

  named @{text "r1_prem"} in the second subproof (Line 14). This local assumption is

  used to solve the goal @{term "P y"} with the help of assumption @{text

  "p1"}.

  \begin{exercise}

  Give the definition for the freshness predicate for lambda-terms. The rules

  for this predicate are:

  \begin{center}\small

  @{prop[mode=Rule] "a\<noteq>b \<Longrightarrow> fresh a (Var b)"}\hspace{5mm}

  @{prop[mode=Rule] "\<lbrakk>fresh a t; fresh a s\<rbrakk> \<Longrightarrow> fresh a (App t s)"}\\[2mm]

  @{prop[mode=Axiom] "fresh a (Lam a t)"}\hspace{5mm}

  @{prop[mode=Rule] "\<lbrakk>a\<noteq>b; fresh a t\<rbrakk> \<Longrightarrow> fresh a (Lam b t)"}

  \end{center}

  From the definition derive the induction principle and the introduction 

  rules. 

  \end{exercise}

  The point of all these examples is to get a feeling what the automatic

  proofs should do in order to solve all inductive definitions we throw at

  them.  This is usually the first step in writing a package. We next explain

  the parsing and typing part of the package.

*}

(*<*)end(*>*)