code: def create_wf(self, text, sent, offset, id=None, length=None):
          
          if id is None:
              n = 1 if self.text_layer is None else len(self.text_layer.idx) + 1
              id = "w{n}".format(**locals())
          if length is None:
              length = len(text)
          new_token = Cwf(type=self.type)
          new_token.set_id(id)
          new_token.set_length(str(length))
          new_token.set_offset(str(offset))
          new_token.set_sent(str(sent))
          new_token.set_text(text)
          self.add_wf(new_token)
          return new_token
docs: Create a new wordform (text token) and add it to the text layer
          @type text: string
          @param text: The text of the token
          @type sent:  int
          @param pos: The sentence of this token
          @type offset: int
          @param morphofeat: The offset of this token
          @type id: string
          @param id: the id of the token, if not given an id wXXX will be created
          @type length: int
          @param length: the length (#characters) of the token,
                         if not given len(text) will be used