Function gdn_fold_trailing 

pub fn gdn_fold_trailing(
    fp: FpUnit,
    q: &[f32],
    log: &[Logged<'_>],
    width: usize,
    older: usize,
) -> [(Vec<f32>, Vec<f32>); 2]

gdn_fold over a log whose oldest older tokens a trailing flush is replaying, in one walk: first through the newer tokens alone, the fold a row the flush has replayed reads, then on through the older ones, the fold every other row reads. The walk is newest first, so the first is where the second has reached after the newer tokens: each is gdn_fold over its own tokens, bit for bit.