Function reciprocal_term 

pub(super) fn reciprocal_term<const N: usize>(
    wave: Family,
    dim: usize,
    k: Jet<N>,
    _: Diffraction<N>,
    r: [Jet<N>; 3],
    eta: Complex,
    split: &Split,
    measure: Jet<N>,
    bound: &mut Rounding<N>,
    plane_integrals: Option<&mut Vec<Option<Complex>>>,
) -> Result<Jet<N>>

One reciprocal summand T(q) of the sum, without the Bloch phase exp(-i q . r) that the caller multiplies in. Every sum shares the argument v = (q . q / k^2 - 1) / (2 eta^2); wave and dim select the sum. 1D spherical sums, whose odd Kambe orders cancel by up to e^(w^2) at w = k rho eta, add the rounding bounds of its value and derivatives in units of epsilon to bound; the other sums keep their relative accuracy.