Checks that ``value`` is an instance of ``DocumentField.type``.
            if it is, then validation on its fields has already been done and
            no further validation is needed.

def validate_wrap(self, value):
        ''' Checks that ``value`` is an instance of ``DocumentField.type``.
            if it is, then validation on its fields has already been done and
            no further validation is needed.
        '''
        if not isinstance(value, self.type):
            self._fail_validation_type(value, self.type)