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

theorem Th59:
  for a being Complex holds r >= 0 implies angle(r,a) = Arg a
proof
  let a be Complex;
  assume r >= 0;
  then Arg r = 0 by COMPTRIG:35;
  hence angle(r,a) = Arg(Rotate(a,-0)) by Def3
    .= Arg a by COMPTRIG:62;
end;
