theorem Th2:
  x in dom cot implies sin.x<>0
proof
  assume x in dom cot;
  then x in dom cos /\ (dom sin \ sin"{0}) by RFUNCT_1:def 1;
  then x in dom sin \ sin"{0} by XBOOLE_0:def 4;
  then x in dom (sin^) by RFUNCT_1:def 2;
  hence thesis by RFUNCT_1:3;
end;
