Function chunk_decay 

pub(crate) fn chunk_decay<B: Backend>(da_blh: F<B, 3>) -> (F<B, 3>, F<B, 4>)

Chunk-local decay: dₜ = exp(a_cum[t]) ([b, h, l]) and the 1-semiseparable mask L[t, j] = exp(a_cum[t] − a_cum[j]) ([b, h, l, l], zero strictly above the diagonal).