Theory InductiveInvariant_examples (Isabelle2005: October 2005)

# Theory InductiveInvariant_examples

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theory InductiveInvariant_examples
imports InductiveInvariant
begin

    (*  ID:         $Id: InductiveInvariant_examples.thy,v 1.4 2005/09/14 20:08:08 wenzelm Exp $
        Author:     Sava Krsti\'{c} and John Matthews
    *)

    header {* Example use if an inductive invariant to solve termination conditions *}

    theory InductiveInvariant_examples imports InductiveInvariant  begin

    text "A simple example showing how to use an inductive invariant
          to solve termination conditions generated by recdef on
          nested recursive function definitions."

    consts g :: "nat => nat"

    recdef (permissive) g "less_than"
      "g 0 = 0"
      "g (Suc n) = g (g n)"

    text "We can prove the unsolved termination condition for
          g by showing it is an inductive invariant."

    recdef_tc g_tc[simp]: g
    apply (rule allI)
    apply (rule_tac x=n in tfl_indinv_wfrec [OF g_def])
    apply (auto simp add: indinv_def split: nat.split)
    apply (frule_tac x=nat in spec)
    apply (drule_tac x="f nat" in spec)
    by auto

    text "This declaration invokes Isabelle's simplifier to
          remove any termination conditions before adding
          g's rules to the simpset."
    declare g.simps [simplified, simp]

    text "This is an example where the termination condition generated
          by recdef is not itself an inductive invariant."

    consts g' :: "nat => nat"
    recdef (permissive) g' "less_than"
      "g' 0 = 0"
      "g' (Suc n) = g' n + g' (g' n)"

    thm g'.simps

    text "The strengthened inductive invariant is as follows
          (this invariant also works for the first example above):"

    lemma g'_inv: "g' n = 0"
    thm tfl_indinv_wfrec [OF g'_def]
    apply (rule_tac x=n in tfl_indinv_wfrec [OF g'_def])
    by (auto simp add: indinv_def split: nat.split)

    recdef_tc g'_tc[simp]: g'
    by (simp add: g'_inv)

    text "Now we can remove the termination condition from
          the rules for g' ."
    thm g'.simps [simplified]

    text {* Sometimes a recursive definition is partial, that is, it
            is only meant to be invoked on "good" inputs. As a contrived
            example, we will define a new version of g that is only
            well defined for even inputs greater than zero. *}

    consts g_even :: "nat => nat"
    recdef (permissive) g_even "less_than"
      "g_even (Suc (Suc 0)) = 3"
      "g_even n = g_even (g_even (n - 2) - 1)"

    text "We can prove a conditional version of the unsolved termination
          condition for @{term g_even} by proving a stronger inductive invariant."

    lemma g_even_indinv: "∃k. n = Suc (Suc (2*k)) ==> g_even n = 3"
    apply (rule_tac D="{n. ∃k. n = Suc (Suc (2*k))}" and x=n in tfl_indinv_on_wfrec [OF g_even_def])
    apply (auto simp add: indinv_on_def split: nat.split)
    by (case_tac ka, auto)

    text "Now we can prove that the second recursion equation for @{term g_even}
          holds, provided that n is an even number greater than two."

    theorem g_even_n: "∃k. n = 2*k + 4 ==> g_even n = g_even (g_even (n - 2) - 1)"
    apply (subgoal_tac "(∃k. n - 2 = 2*k + 2) & (∃k. n = 2*k + 2)")
    by (auto simp add: g_even_indinv, arith)

    text "McCarthy's ninety-one function. This function requires a
          non-standard measure to prove termination."

    consts ninety_one :: "nat => nat"
    recdef (permissive) ninety_one "measure (%n. 101 - n)"
      "ninety_one x = (if 100 < x
                         then x - 10
                         else (ninety_one (ninety_one (x+11))))"

    text "To discharge the termination condition, we will prove
          a strengthened inductive invariant:
             S x y == x < y + 11"

    lemma ninety_one_inv: "n < ninety_one n + 11"
    apply (rule_tac x=n in tfl_indinv_wfrec [OF ninety_one_def])
    apply force
    apply (auto simp add: indinv_def measure_def inv_image_def)
    apply (frule_tac x="x+11" in spec)
    apply (frule_tac x="f (x + 11)" in spec)
    by arith

    text "Proving the termination condition using the
          strengthened inductive invariant."

    recdef_tc ninety_one_tc[rule_format]: ninety_one
    apply clarify
    by (cut_tac n="x+11" in ninety_one_inv, arith)

    text "Now we can remove the termination condition from
          the simplification rule for @{term ninety_one}."

    theorem def_ninety_one:
    "ninety_one x = (if 100 < x
                       then x - 10
                       else ninety_one (ninety_one (x+11)))"
    by (subst ninety_one.simps,
        simp add: ninety_one_tc measure_def inv_image_def)

    end

lemma g'_inv:

      g' n = 0

lemma g_even_indinv:

      ∃k. n = Suc (Suc (2 * k)) ==> g_even n = 3

theorem g_even_n:

      ∃k. n = 2 * k + 4 ==> g_even n = g_even (g_even (n - 2) - 1)

lemma ninety_one_inv:

      n < ninety_one n + 11

theorem def_ninety_one:

      ninety_one x = (if 100 < x then x - 10 else ninety_one (ninety_one (x + 11)))