A way to synchronise items with safe merge conflicts.
If the collection that you are syncing has immutable items, you will be better off using mergeless instead.
Situation: Multiple clients (for example a note synchronisation app) want to sync with a server storing state for them. Goal: Get clients to agree on the collection of items and prevent that one client accidentally overrides a value saved by another client. Solution: Store a server-side name and monotonically increasing number (revision number of the data, or current time) for each number.
See the following blogposts for (many) more details:
or the talk at Haskellerz: