Function fold 

pub fn fold(
    bytes: &[u8],
    emit: impl FnMut(i64, u64) -> Result<()>,
) -> Result<usize>

Visits the values of one encoded chunk with their runtime row counts. Sparse chunks written with sorted exception positions need no per-chunk count map. An older or malformed chunk with repeated positions keeps the decoder’s last-write-wins behavior.

Errors

As decode, or if the callback rejects a count.