protected function validateImmutable($field, $val, $required = true)
    {
        if ($this->getInitial($field) && $this->valueDiffersFromInitial($field, $val)) {
            $this->setError($field, 'immutable', [
                "title" => "`$field` is Immutable",
                "detail" => "You can't change the `$field` field of this resource once it's been set.",
            ]);
            return false;
        } else {
            $this->clearError($field, 'immutable');
            return true;
        }
    }