code: def test_message_policy_used_by_as_string(self):
      msg = self._make_msg(
          "Subject: test\nTo: foo\n\n",
          policy=email.policy.default.clone(linesep="X"),
      )
      self.assertEqual(msg.as_string(), "Subject: testXTo: fooXX")
  
imports: import io
  import types
  import textwrap
  import unittest
  import email.errors
  import email.policy
  import email.parser
  import email.generator
  import email.message
  from email import headerregistry
label_desc: Use image node_id 11 for calling the TestPolicyPropagation obj's underlying member method code with example usage: obj.test_message_policy_used_by_as_string() without return types