theorem Th62:
  A\orB\imp(\notA\impB) in F
  proof
A1: A\imp(\notA\impB) in F by Th56;
A2: B\imp(\notA\impB) in F by Def38;
    A\imp(\notA\impB)\imp((B\imp(\notA\impB))\imp(A\orB\imp(\notA\impB))) in F
    by Def38; then
    (B\imp(\notA\impB))\imp(A\orB\imp(\notA\impB)) in F by A1,Def38;
    hence thesis by A2,Def38;
  end;
