class_docstr: Main data class: encapsulate list of utterances, mentions and speakers
  Process utterances to extract mentions and pre-compute mentions features
docstr: Return a specific mention (not utterance) 
docstr_tok: ["Return","a","specific","mention","(","not","utterance",")"]
code: def __getitem__(self, key):
      """Return a specific mention (not utterance)"""
      return self.mentions[key]
  
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 5 for calling the Document obj's underlying member method code with example usage: obj.__getitem__(key) and returns: self