fn plane<const N: usize>( label: WaveLabel, args: [Jet<N>; 6], helicity: bool, ) -> Result<[Jet<N>; 3]>
Plane wave p exp(i k·r) at (kx, ky, kz, x, y, z), with the chain rule through the polarization p(k).
p exp(i k·r)
(kx, ky, kz, x, y, z)
p(k)