Removes the tokens of the given sentence
        @type sentence_id: string
        @param sentence_id: the sentence identifier

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)