code: def add_wf(self,wf_obj):
          
          if wf_obj.get_id() in self.idx:
              raise ValueError("Text node (wf) with id {} already exists!"
                               .format(wf_obj.get_id()))
          self.node.append(wf_obj.get_node())
          self.idx[wf_obj.get_id()] = wf_obj
docs: Adds a token object to the text layer
          @type wf_obj: L{Cwf}
          @param wf_obj: token object