reserve a, b, c, d, x, y, z for Complex;
reserve r for Real;

theorem Th57:
  Rotate(a-b,r) = Rotate(a,r)-Rotate(b,r)
proof
  thus Rotate(a-b,r) = Rotate(a,r)+Rotate(-b,r) by Th55
    .= Rotate(a,r)-Rotate(b,r) by Th56;
end;
