code: def build(self):
          
          for i in range(2):
              self._molecules.append(
                  self.make_helix(self.aas[i], self.axis_distances[i],
                                  self.z_shifts[i], self.phis[i], self.splays[i],
                                  self.off_plane[i]))
          return
docs: Builds a `HelixPair` using the defined attributes.