def is_feasible_i(self, x, i):
        """return True if value ``x`` is in the invertible domain of
        variable ``i``

        """
        lb = self._lb[self._index(i)]
        ub = self._ub[self._index(i)]
        al = self._al[self._index(i)]
        au = self._au[self._index(i)]
        return lb - al < x < ub + au