theorem Lem2:
  x =+=> y implies x =*=> y
  proof
    assume
A1: x =+=> y;
    consider z such that
A2: x ==> z & z =*=> y by A1;
A3: x =*=> z by A2,Th2;
    thus x =*=> y by A2,A3,Th3;
  end;
