docs: def bind(self, isnap, istep):
          """Register the isnap / istep correspondence.
  
          Users of :class:`StagyyData` should not use this method.
  
          Args:
              isnap (int): snapshot index.
              istep (int): time step index.
          """
          self._isteps[isnap] = istep
          self.sdat.steps[istep].isnap = isnap