'Turns pen clockwise by angle.'

def _rotate(self, angle):

    if self.undobuffer:
        self.undobuffer.push(('rot', angle, self._degreesPerAU))
    angle *= self._degreesPerAU
    neworient = self._orient.rotate(angle)
    tracing = self.screen._tracing
    if ((tracing == 1) and (self._speed > 0)):
        anglevel = (3.0 * self._speed)
        steps = (1 + int((abs(angle) / anglevel)))
        delta = ((1.0 * angle) / steps)
        for _ in range(steps):
            self._orient = self._orient.rotate(delta)
            self._update()
    self._orient = neworient
    self._update()
