(*  Title:      FOL/ex/If.thy
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1991  University of Cambridge
*)

section ‹First-Order Logic: the 'if' example›

theory If
imports FOL
begin

definition "if" :: ‹[o,o,o]=>o›
  where ‹if(P,Q,R) ≡ P ∧ Q ∨ ¬ P ∧ R›

lemma ifI: ‹[P ==> Q; ¬ P ==> R] ==> if(P,Q,R)›
  unfolding if_def by blast

lemma ifE: ‹[if(P,Q,R); [P; Q] ==> S; [¬ P; R] ==> S] ==> S›
  unfolding if_def by blast

lemma if_commute: ‹if(P, if(Q,A,B), if(Q,C,D)) ⟷ if(Q, if(P,A,C), if(P,B,D))›
  apply (rule iffI)
  apply (erule ifE)
  apply (erule ifE)
  apply (rule ifI)
  apply (rule ifI)
  oops

text‹Trying again from the beginning in order to use ‹blast››
declare ifI [intro!]
declare ifE [elim!]

lemma if_commute: ‹if(P, if(Q,A,B), if(Q,C,D)) ⟷ if(Q, if(P,A,C), if(P,B,D))›
  by blast


lemma ‹if(if(P,Q,R), A, B) ⟷ if(P, if(Q,A,B), if(R,A,B))›
  by blast

text‹Trying again from the beginning in order to prove from the definitions›
lemma ‹if(if(P,Q,R), A, B) ⟷ if(P, if(Q,A,B), if(R,A,B))›
  unfolding if_def by blast


text ‹An invalid formula. High-level rules permit a simpler diagnosis.›
lemma ‹if(if(P,Q,R), A, B) ⟷ if(P, if(Q,A,B), if(R,B,A))›
  apply auto
    ― ‹The next step will fail unless subgoals remain›
  apply (tactic all_tac)
  oops

text ‹Trying again from the beginning in order to prove from the definitions.›
lemma ‹if(if(P,Q,R), A, B) ⟷ if(P, if(Q,A,B), if(R,B,A))›
  unfolding if_def
  apply auto
    ― ‹The next step will fail unless subgoals remain›
  apply (tactic all_tac)
  oops

end