
theorem
  for a be Real holds sin (2*PI-a) = - sin a
  proof
    let a be Real;
    sin (2* PI - a) = 0 * cos a - cos (2*PI) * sin a by SIN_COS:77,SIN_COS:82
    .= - sin a by SIN_COS:77;
    hence thesis;
  end;
