class_docstr: vocabulary 
docstr: update vocab statis
docstr_tok: ["update","vocab","statis"]
code: def add(self, word):
      """update vocab statis"""
      if word not in self._mapping:
          self._mapping[word] = len(self._mapping)
      self._freq[word] += 1
  
file_path: delta/delta/data/utils/vocabulary.py
imports: import copy
  import collections
label_desc: Use image node_id 3 for calling the Vocabulary obj's underlying member method code with example usage: obj.add(word) without return types