Neutrino direction in polar coordinates.

    ``phi``, ``theta`` is the opposite of ``zenith``, ``azimuth``.

    Angles in radians.

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)