docs: def remove_tokens_of_sentence(self,sentence_id):
          """
          Removes the tokens of the given sentence
          @type sentence_id: string
          @param sentence_id: the sentence identifier  
          """
          nodes_to_remove = set()
          for wf in self:
              if wf.get_sent() == sentence_id:
                  nodes_to_remove.add(wf.get_node())
          
          for node in nodes_to_remove:
              self.node.remove(node)