This is the environment implementation of :meth:`BaseKerning.scaleBy`.
        **factor** will be a ``tuple``.

        Subclasses may override this method.

def _scale(self, factor):
        """
        This is the environment implementation of :meth:`BaseKerning.scaleBy`.
        **factor** will be a ``tuple``.

        Subclasses may override this method.
        """
        factor = factor[0]
        for k, v in self.items():
            v *= factor
            self[k] = v