class_docstr: Main data class: encapsulate list of utterances, mentions and speakers
  Process utterances to extract mentions and pre-compute mentions features
docstr: Iterate over mentions (not utterances) 
docstr_tok: ["Iterate","over","mentions","(","not","utterances",")"]
code: def __iter__(self):
      """Iterate over mentions (not utterances)"""
      for mention in self.mentions:
          yield mention
  
imports: import re
  import io
  from six import string_types, integer_types
  from spacy.tokens import Span, Token
  from neuralcoref.train.compat import unicode_
  from neuralcoref.train.utils import encode_distance, parallel_process
  import spacy
  import numpy
label_desc: Use image node_id 6 for calling the Document obj's underlying member method code with example usage: obj.__iter__() without return types