Function to_fetch 

fn to_fetch(
    found: impl IntoIterator<Item = u32>,
    last_uid: u32,
    root: &Path,
    validity: u32,
) -> (Vec<u32>, u32)

The UIDs to fetch of those found, oldest first, and the newest of those here already (else last_uid). Here already: a sync stopped between storing mail and saving where it stopped, or whose state was lost, finds them again, and they are not stored twice.