<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tseries(self):
 """Time series data. This is a :class:`pandas.DataFrame` with istep as index and variable names as columns. """

        if self._stagdat['tseries'] is UNDETERMINED:
            timefile = self.filename('TimeSeries.h5')
            self._stagdat['tseries'] = stagyyparsers.time_series_h5(
                timefile, list(phyvars.TIME.keys()))
            if self._stagdat['tseries'] is not None:
                return self._stagdat['tseries']
            timefile = self.filename('time.dat')
            if self.hdf5 and not timefile.is_file():
                # check legacy folder as well
                timefile = self.filename('time.dat', force_legacy=True)
            self._stagdat['tseries'] = stagyyparsers.time_series(
                timefile, list(phyvars.TIME.keys()))
        return self._stagdat['tseries']