Call the method `mtd` on both sides of the equation

        That is, the left-hand-side and right-hand-side are replaced by::

            lhs=lhs.<mtd>(*args, **kwargs)
            rhs=rhs.<mtd>(*args, **kwargs)

        The `cont` and `tag` parameters are as in :meth:`apply`.

def apply_mtd(self, mtd, *args, cont=False, tag=None, **kwargs):
        """Call the method `mtd` on both sides of the equation

        That is, the left-hand-side and right-hand-side are replaced by::

            lhs=lhs.<mtd>(*args, **kwargs)
            rhs=rhs.<mtd>(*args, **kwargs)

        The `cont` and `tag` parameters are as in :meth:`apply`.
        """
        new_lhs = getattr(self.lhs, mtd)(*args, **kwargs)
        if new_lhs == self.lhs and cont:
            new_lhs = None
        new_rhs = getattr(self.rhs, mtd)(*args, **kwargs)
        new_tag = tag
        return self._update(new_lhs, new_rhs, new_tag, cont)