docs: def neutrino_to_source_direction(phi, theta, radian=True):
      """Flip the direction.
  
      Parameters
      ==========
      phi, theta: neutrino direction
      radian: bool [default=True]
          receive + return angles in radian? (if false, use degree)
      """
      phi = np.atleast_1d(phi).copy()
      theta = np.atleast_1d(theta).copy()
      if not radian:
          phi *= np.pi / 180
          theta *= np.pi / 180
      assert np.all(phi <= 2 * np.pi)
      assert np.all(theta <= np.pi)
      azimuth = (phi + np.pi) % (2 * np.pi)
      zenith = np.pi - theta
      if not radian:
          azimuth *= 180 / np.pi
          zenith *= 180 / np.pi
      return azimuth, zenith