Function drain_side 

fn drain_side<D, T, R>(
    head: &mut Column<(D, T, R)>,
    pos: &mut usize,
    list: &mut IntoIter<Column<(D, T, R)>>,
    result: &mut Column<(D, T, R)>,
    output: &mut Vec<Column<(D, T, R)>>,
    stash: &mut Vec<Column<(D, T, R)>>,
)
where
    D: Columnar,
    for<'a> Ref<'a, D>: Copy + Ord,
    T: Columnar + Default + Clone + PartialOrder,
    for<'a> Ref<'a, T>: Copy + Ord,
    R: Columnar + Default + Semigroup + for<'a> Semigroup<Ref<'a, R>>,

Drain remaining items from one side into result / output.

Copies the partially-consumed head into result via merge_from’s 1-input path, then appends remaining full chunks directly to output without per-element copy.