Module I_USet

Note: Removed elements can never be added again. This is because of how USet handles the value operation (see below).

Supports add, remove, lookup and merge operations. See Immutable_types.RSet. Contains two separate instances of a regular Map, one for adding (add_s) and the other for removing (remove_s).

add el t adds el to add_s.

remove el t adds el to remove_s

Both lookup and value operate on the set difference add_s \ remove_s.

merge a b calls merges add_s_a with add_s_b and the same for b