def METHOD_NAME(self):
    """
    The residual of the constraint.
    Returns
    -------
    float
    """
    expr_val = np.array(self.expre.value).flatten()
    vec_val = self.vec.value
    resids = [np.min(np.abs(val - vec_val)) for val in expr_val]
    res = max(resids)
    return res