docs: def phi(v):
      """Neutrino direction in polar coordinates.
  
      ``phi``, ``theta`` is the opposite of ``zenith``, ``azimuth``.
  
      Angles in radians.
      """
      v = np.atleast_2d(v)
      dir_x = v[:, 0]
      dir_y = v[:, 1]
      return phi_separg(dir_x, dir_y)