This is the environment implementation of
        :meth:`BaseAnchor.transformBy`.

        **matrix** will be a :ref:`type-transformation`.
        that has been normalized with
        :func:`normalizers.normalizeTransformationMatrix`.

        Subclasses may override this method.

def _transformBy(self, matrix, **kwargs):
        """
        This is the environment implementation of
        :meth:`BaseAnchor.transformBy`.

        **matrix** will be a :ref:`type-transformation`.
        that has been normalized with
        :func:`normalizers.normalizeTransformationMatrix`.

        Subclasses may override this method.
        """
        t = transform.Transform(*matrix)
        x, y = t.transformPoint((self.x, self.y))
        self.x = x
        self.y = y