Function solid_jet 

pub(super) fn solid_jet<const N: usize>(
    l: i32,
    m: i32,
    r: [Jet<N>; 3],
) -> Jet<N>

The solid harmonic r^l P_l^m(cos theta) e^(i m phi) of the jet vector r, with its derivatives by the chain rule through the Cartesian gradient. Times harmonic_normalization(l, m) it is |r|^l Y_lm(r / |r|), a polynomial in r that stays regular at r = 0.