theorem Th102:
  A\impB in F implies C\impA\imp(C\impB) in F
  proof
    C\impA\imp(A\impB\imp(C\impB)) in F by Th39;
    then A\impB\imp(C\impA\imp(C\impB)) in F by Th38;
    hence thesis by Def38;
  end;
