<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_xml_string_with_self_contained_assertion_within_encrypted_assertion( self, assertion_tag):
 """ Makes a encrypted assertion only containing self contained namespaces. :param assertion_tag: Tag for the assertion to be transformed. :return: A new samlp.Resonse in string representation. """

        prefix_map = self.get_prefix_map(
            [self.encrypted_assertion._to_element_tree().find(assertion_tag)])

        tree = self._to_element_tree()

        self.set_prefixes(
            tree.find(
                self.encrypted_assertion._to_element_tree().tag).find(
                assertion_tag), prefix_map)

        return ElementTree.tostring(tree, encoding="UTF-8").decode('utf-8')