position will be a valid position (x, y).
        angle will be a valid angle.
        name will be a valid guideline name or None.
        color will be a valid color or None .
        identifier will be a valid, nonconflicting identifier.

        This must return the new guideline.

        Subclasses may override this method.

def _appendGuideline(self, position, angle, name=None, color=None, identifier=None, **kwargs):
        """
        position will be a valid position (x, y).
        angle will be a valid angle.
        name will be a valid guideline name or None.
        color will be a valid color or None .
        identifier will be a valid, nonconflicting identifier.

        This must return the new guideline.

        Subclasses may override this method.
        """
        self.raiseNotImplementedError()