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

        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