Function direct_term 

fn direct_term<const N: usize>(
    wave: Family,
    k: Jet<N>,
    r: [Jet<N>; 3],
) -> Result<Jet<N>>

One image of a direct sum: the outgoing wave h_l^(1)(k |r|) Y_lm(r / |r|) or H_m^(1)(k |r|) e^(i m phi) at the displacement r = -shift - R (upstream _fsw* and _fcw*). Values take special::bessel; jets take the radial jets, whose first derivative the chain rule carries. The spherical harmonic is harmonic_normalization(l, m) times the solid harmonic of the unit vector.