def set_timestamp(self,timestamp=None):
        """
        Set the timestamp of the linguistic processor, set to None for the current time
        @type timestamp:string
        @param timestamp: version of the linguistic processor
        """
        if timestamp is None:
            import time
            timestamp = time.strftime('%Y-%m-%dT%H:%M:%S%Z')
        self.node.set('timestamp',timestamp)