Function tile_descriptions 

pub fn tile_descriptions<T: Timestamp + Lattice>(
    lower: &Antichain<T>,
    upper: &Antichain<T>,
    held: &[T],
) -> (Vec<Description<T>>, Vec<T>, Vec<Option<usize>>)

Cuts the interval [lower, upper) into consecutive batch descriptions along held, which must be sorted: the i-th cut point is the frontier formed by inserting held[i+1..] into upper, so description i covers the part of the interval not greater-or-equal any held time after held[i] (and not covered by an earlier description). Descriptions whose interval is empty are skipped. Returns the descriptions, the held time associated with each, and, per held index, the index of its description (None if skipped). A batch built to description i can be committed at the capability held[i].