docs: def strands(self):
          """Generates a new `Assembly` containing only the β-strands.
  
          Notes
          -----
          Metadata is not currently preserved from the parent object.
  
          Returns
          -------
          strand_assembly : ampal.Protein
              `Assembly` containing only the β-strands of the original `Assembly`.
          """
          strand_molecules = list(itertools.chain(
              *[p.strands._molecules for p in self._molecules if hasattr(p, 'strands')]))
          strand_assembly = Assembly(
              molecules=strand_molecules, assembly_id=self.id)
          return strand_assembly