pub fn program_fourth_contracted<const K: usize, P: RowProgram<K> + ?Sized>(
prog: &P,
row: usize,
dir_u: &[f64; K],
dir_v: &[f64; K],
) -> Result<[[f64; K]; K], String>Expand description
Derive the row_fourth_contracted(u, v) channel Σ_{cd} ℓ_{abcd} u_c v_d
from a RowProgram at the two-seed scalar crate::jet_scalar::TwoSeed,
WITHOUT materialising the dense t4.