Function next_mint 

fn next_mint(
    current_upper: &Antichain<Timestamp>,
    desired_frontier: &Antichain<Timestamp>,
    remap_upper: &Antichain<Timestamp>,
    committed: Option<Timestamp>,
    lookahead: Option<u64>,
) -> Option<Mint>

The next thing the minter emits, or None when there is nothing to do.

A description is derived from the frontier, which certifies that everything below it has arrived. An outstanding committed ceiling suppresses that until the frontier reaches the ceiling, which is what collapses a whole snapshot and the catch-up behind it into one description.

• current_upper: The upper of the last minted description, initialized from the shard. Any data before this has already been processed.
• desired_frontier: Data input frontier. This frontier advances when the reclocked data stream advances.
• remap_upper: The frontier of the remap shard, which advances on each successful probe.
• committed (Optional): This is the current value of the ceiling. If set, the upper of a minted description will be at or beyond this.
• lookahead (Optional): If present, desired_frontier is pinned to as_of (e.g. due to snapshot), so we advance committed as remap_upper + lookahead to consolidate in-flight data into one shared batch.