Returns the timex object for the supplied identifier
        @type timex_id: string
        @param timex_id: timex identifier

def get_timex(self, timex_id):
        """
        Returns the timex object for the supplied identifier
        @type timex_id: string
        @param timex_id: timex identifier
        """
        if timex_id in self.idx:
            return Ctime(self.idx[timex_id])
        else:
            return None