Module type Parameters.VALUE
The values associated with each equivalence class in the union-find. Values have a lattice structure. For flexibility, the lattice value operations also take the a NODE (often the representative) as an extra argument. It can be ignored if irrelevant.
the type of NODE.
incl x y is true if x is included (smaller than) y (i.e. x = meet x y or y = join x y).
Union of values, only required for the joins. Can return None for a top value that does not need to be stored.