docs: def geom(self):
          """Geometry information.
  
          :class:`_Geometry` instance holding geometry information. It is
          issued from binary files holding field information. It is set to
          None if not available for this time step.
          """
          if self._header is UNDETERMINED:
              binfiles = self.step.sdat.binfiles_set(self.step.isnap)
              if binfiles:
                  self._header = stagyyparsers.fields(binfiles.pop(),
                                                      only_header=True)
              elif self.step.sdat.hdf5:
                  xmf = self.step.sdat.hdf5 / 'Data.xmf'
                  self._header, _ = stagyyparsers.read_geom_h5(xmf,
                                                               self.step.isnap)
              else:
                  self._header = None
          if self._geom is UNDETERMINED:
              if self._header is None:
                  self._geom = None
              else:
                  self._geom = _Geometry(self._header, self.step.sdat.par)
          return self._geom