class_docstr: A base class for common vocabulary operations. Each subclass should at least 
  implement its own build_vocab() function.
code: def normalize_unit(self, unit):
      # be sure to look in subclasses for other normalization being done
      # especially PretrainWordVocab
      if unit is None:
          return unit
      if self.lower:
          return unit.lower()
      return unit
  
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 5 for calling the BaseVocab obj's underlying member method code with example usage: obj.normalize_unit(unit) and returns: unit, unit, unit