theorem Th52:
  A\orA\impA in F
  proof
A1: A\impA\imp(A\impA\imp(A\orA\impA)) in F by Def38;
A2: A\impA in F by Th34;
    then A\impA\imp(A\orA\impA) in F by A1,Def38;
    hence thesis by A2,Def38;
  end;
