code: def test_callback_forward_index(self):
      def myreplace(exc):
          return ("REPLACED", exc.end + 2)
  
      codecs.register_error("test.cjktest", myreplace)
      self.assertEqual(
          self.encode(
              "abcd" + self.unmappedunicode + "efgh", "test.cjktest"
          ),
          (b"abcdREPLACEDgh", 9),
      )
  
imports: import codecs
  import os
  import re
  import sys
  import unittest
  from http.client import HTTPException
  from test import support
  from io import BytesIO
label_desc: Use image node_id 11 for calling the TestBase obj's underlying member method code with example usage: obj.test_callback_forward_index() and returns: str,  unknown