theorem Th39:
  for z for s,t be Complex holds s*z,t*z are_commutative
proof
  let z;
  let s,t be Complex;
  (s*z) *(t*z) =(s*t)*(z*z) by CLOPBAN3:38
    .=(t*z)*(s*z) by CLOPBAN3:38;
  hence thesis by LOPBAN_4:def 1;
end;
