theorem Th39:
  for c being Complex
  for s being Real holds Rotate(c,s) = Rotate(c,s+2*PI*i)
  proof
    let c be Complex;
    let s be Real;
    cos (s+Arg c) = cos (s+Arg c+2*PI*i) &
    sin (s+Arg c) = sin (s+Arg c+2*PI*i) by COMPLEX2:8,9;
    hence thesis;
  end;
