code: def _extract_substrs(self, txt_input, start, end):
      extracted = []
      start = self.evaluate(start)
      end = self.evaluate(end)
      txt_input = txt_input.encode("utf-8")
      for i in range(start.shape[1]):
          pre_norm_start = int(start[0][i])
          pre_norm_end = int(end[0][i])
          extracted.append(txt_input[pre_norm_start:pre_norm_end])
      return extracted
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import parameterized
  from tensorflow.python.framework import errors
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops.ragged import ragged_factory_ops
  from tensorflow.python.platform import test
  from tensorflow_text.python.ops import normalize_ops
label_desc: Use image node_id 1 for calling the FindSourceOffsetsTest obj's underlying member method code with example usage: obj._extract_substrs(txt_input, start, end) and returns: extracted