Generates new `Assembly` containing just α-helices.

        Notes
        -----
        Metadata is not currently preserved from the parent object.

        Returns
        -------
        hel_assembly : ampal.Protein
            `Assembly` containing only the α-helices of the original `Assembly`.

def helices(self):
        """Generates new `Assembly` containing just α-helices.

        Notes
        -----
        Metadata is not currently preserved from the parent object.

        Returns
        -------
        hel_assembly : ampal.Protein
            `Assembly` containing only the α-helices of the original `Assembly`.
        """
        hel_molecules = list(itertools.chain(
            *[p.helices._molecules
              for p in self._molecules if hasattr(p, 'helices')]))
        hel_assembly = Assembly(molecules=hel_molecules, assembly_id=self.id)
        return hel_assembly