def test_subdim_middle(self, opt):
        """
        Tests that instantiating SubDimensions using the classmethod
        constructors works correctly.
        """
        grid = Grid(shape=(4, 4, 4))
        x, y, z = grid.dimensions
        t = grid.stepping_dim  # noqa

        u = TimeFunction(name='u', grid=grid)  # noqa
        xi = SubDimension.middle(name='xi', parent=x,
                                 thickness_left=1,
                                 thickness_right=1)
        eqs = [Eq(u.forward, u + 1)]
        eqs = [e.subs(x, xi) for e in eqs]

        op = Operator(eqs, opt=opt)

        u.data[:] = 1.0
        op.apply(time_M=1)
        assert np.all(u.data[1, 0, :, :] == 1)
        assert np.all(u.data[1, -1, :, :] == 1)
        assert np.all(u.data[1, 1:3, :, :] == 2)