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

theorem Th62:
  a <> 0 & b <> 0 & Arg a = Arg b implies Arg -a = Arg -b
proof
  assume a <> 0 & b <> 0 & Arg a = Arg b;
  then Arg Rotate(a,PI) = Arg Rotate(b,PI) by Th60;
  then Arg -a = Arg Rotate(b,PI) by Th58;
  hence thesis by Th58;
end;
