code: def tokenize_words(self):
          
          if not self.is_tagged(SENTENCES):
              self.tokenize_sentences()
          tok = self.__word_tokenizer
          text = self.text
          dicts = []
          for sentence in self[SENTENCES]:
              sent_start, sent_end = sentence[START], sentence[END]
              sent_text = text[sent_start:sent_end]
              spans = tok.span_tokenize(sent_text)
              for start, end in spans:
                  dicts.append({START: start+sent_start, END: end+sent_start, TEXT: sent_text[start:end]})
          self[WORDS] = dicts
          return self
docs: Apply word tokenization and create ``words`` layer.
  
          Automatically creates ``paragraphs`` and ``sentences`` layers.