Theory Util

(*  Title:      HOL/Proofs/Extraction/Util.thy
    Author:     Stefan Berghofer, TU Muenchen
*)

section ‹Auxiliary lemmas used in program extraction examples›

theory Util
imports Main
begin

text ‹Decidability of equality on natural numbers.›

lemma nat_eq_dec: "⋀n::nat. m = n ∨ m ≠ n"
  apply (induct m)
  apply (case_tac n)
  apply (case_tac [3] n)
  apply (simp only: nat.simps, iprover?)+
  done

text ‹
  Well-founded induction on natural numbers, derived using the standard
  structural induction rule.
›

lemma nat_wf_ind:
  assumes R: "⋀x::nat. (⋀y. y < x ⟹ P y) ⟹ P x"
  shows "P z"
proof (rule R)
  show "⋀y. y < z ⟹ P y"
  proof (induct z)
    case 0
    then show ?case by simp
  next
    case (Suc n y)
    from nat_eq_dec show ?case
    proof
      assume ny: "n = y"
      have "P n"
        by (rule R) (rule Suc)
      with ny show ?case by simp
    next
      assume "n ≠ y"
      with Suc have "y < n" by simp
      then show ?case by (rule Suc)
    qed
  qed
qed

text ‹Bounded search for a natural number satisfying a decidable predicate.›

lemma search:
  assumes dec: "⋀x::nat. P x ∨ ¬ P x"
  shows "(∃x<y. P x) ∨ ¬ (∃x<y. P x)"
proof (induct y)
  case 0
  show ?case by simp
next
  case (Suc z)
  then show ?case
  proof
    assume "∃x<z. P x"
    then obtain x where le: "x < z" and P: "P x" by iprover
    from le have "x < Suc z" by simp
    with P show ?case by iprover
  next
    assume nex: "¬ (∃x<z. P x)"
    from dec show ?case
    proof
      assume P: "P z"
      have "z < Suc z" by simp
      with P show ?thesis by iprover
    next
      assume nP: "¬ P z"
      have "¬ (∃x<Suc z. P x)"
      proof
        assume "∃x<Suc z. P x"
        then obtain x where le: "x < Suc z" and P: "P x" by iprover
        have "x < z"
        proof (cases "x = z")
          case True
          with nP and P show ?thesis by simp
        next
          case False
          with le show ?thesis by simp
        qed
        with P have "∃x<z. P x" by iprover
        with nex show False ..
      qed
      then show ?case by iprover
    qed
  qed
qed

end