<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def backbone(self):
 """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. """

        bb_poly = Polypeptide([x.backbone for x in self._monomers], self.id)
        return bb_poly