reserve th, th1, th2, th3 for Real;

theorem
  cot(-th) = -cot(th)
proof
  cot(-th) =cos(th)/sin(-th) by SIN_COS:31
    .=cos(th)/(-sin(th)) by SIN_COS:31
    .=-cot(th) by XCMPLX_1:188;
  hence thesis;
end;
