Function generate_with_overlay_inner 

pub fn generate_with_overlay_inner<'g, I: Iterator<Item = OwnedRow> + 'g>(
    inner: I,
    overlays: Overlays,
    sort: Sort,
    reverse: bool,
) -> RowFlow<'g>

Splice already-computed overlays into an ordered row stream. Exposed for direct testing (the JS generateWithOverlayInner suite). sort/reverse are the index comparator.