code: def test_insert_field_before_index(self):
      # test insert before index and mask
      ins_bef = mrc_sample.insert_field_before_index(
          "context", 2, "textflint"
      )
      self.assertTrue(ins_bef.is_legal())
      self.assertEqual(
          "Super Bowl textflint 50 was an American football game to "
          "determine the champion "
          "of the National Football League (NFL) for the 2015 season.",
          ins_bef.get_sentences("context")[0],
      )
  
file_path: textflint/test/input/component/sample/test_mrc_sample.py
imports: import unittest
  from textflint.input.component.sample.mrc_sample import MRCSample
label_desc: Use image node_id 4 for calling the TestMRCSample obj's underlying member method code with example usage: obj.test_insert_field_before_index() without return types