Return a copy of the equation with a new `tag`

def set_tag(self, tag):
        """Return a copy of the equation with a new `tag`"""
        return Eq(
            self._lhs, self._rhs, tag=tag,
            _prev_lhs=self._prev_lhs, _prev_rhs=self._prev_rhs,
            _prev_tags=self._prev_tags)