code: def backbone(self):
          
          bb_molecules = [
              p.backbone for p in self._molecules if hasattr(p, 'backbone')]
          bb_assembly = Assembly(bb_molecules, assembly_id=self.id)
          return bb_assembly
docs: Generates a new `Assembly` containing only the backbone atoms.
  
          Notes
          -----
          Metadata is not currently preserved from the parent object.
          Sequence data is retained, but only the main chain atoms are
          retained.
  
          Returns
          -------
          bb_assembly : ampal.Protein
              `Assembly` containing only the backbone atoms of the original
              `Assembly`.