theorem Th36:
  A\orB\impB\orA in F
  proof
A1: (A\impB\orA)\imp((B\impB\orA)\imp(A\orB\impB\orA)) in F by Def38;
A2: (A\impB\orA) in F & B\impB\orA in F by Def38;
    (B\impB\orA)\imp(A\orB\impB\orA) in F by A1,A2,Def38;
    hence thesis by A2,Def38;
  end;
