theorem Th53:
  A\impA\andA in F
  proof
A1: A\impA\imp(A\impA\imp(A\impA\andA)) in F by Th49;
A2: A\impA in F by Th34;
    then A\impA\imp(A\impA\andA) in F by A1,Def38;
    hence thesis by A2,Def38;
  end;
