class_docstr: A base class for common vocabulary operations. Each subclass should at least 
  implement its own build_vocab() function.
code: def id2unit(self, id):
      return self._id2unit[id]
  
imports: from copy import copy
  from collections import Counter, OrderedDict
  from collections.abc import Iterable
  import os
  import pickle
label_desc: Use image node_id 7 for calling the BaseVocab obj's underlying member method code with example usage: obj.id2unit(id) and returns: self