docs: def theta(v):
      """Neutrino direction in polar coordinates.
  
      Downgoing event: theta = 180deg
      Horizont: 90deg
      Upgoing: theta = 0
  
      Angles in radians.
      """
      v = np.atleast_2d(v)
      dir_z = v[:, 2]
      return theta_separg(dir_z)