code: def backbone(self):
          
          bb_poly = Polypeptide([x.backbone for x in self._monomers], self.id)
          return bb_poly
docs: Returns a new `Polymer` 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_poly : Polypeptide
              Polymer containing only the backbone atoms of the original
              Polymer.