code: def primitive(self):
          
          cas = self.get_reference_coords()
          primitive_coords = make_primitive_extrapolate_ends(
              cas, smoothing_level=self.sl)
          primitive = Primitive.from_coordinates(primitive_coords)
          primitive.relabel_monomers([x.id for x in self])
          primitive.id = self.id
          primitive.parent = self
          return primitive
docs: Primitive of the backbone.
  
          Notes
          -----
          This is the average of the positions of all the CAs in frames
          of `sl` `Residues`.