code: def _check_perpendicular_r2_axis(self, axis):
          
          min_set = self._get_smallest_set_not_on_axis(axis)
          for s1, s2 in itertools.combinations(min_set, 2):
              test_axis = np.cross(s1.coords - s2.coords, axis)
              if np.linalg.norm(test_axis) > self.tol:
                  op = SymmOp.from_axis_angle_and_translation(test_axis, 180)
                  r2present = self.is_valid_op(op)
                  if r2present:
                      self.symmops.append(op)
                      self.rot_sym.append((test_axis, 2))
                      return True
docs: Checks for R2 axes perpendicular to unique axis.  For handling
          symmetric top molecules.