SWI-Prolog 8.2.4 Reference Manual: Section A.23

## A.23 library(nb_set): Non-backtrackable set

The library `library(nb_set)` defines non-backtrackable sets, implemented as binary trees. The sets are represented as compound terms and manipulated using nb_setarg/3. Non-backtrackable manipulation of data structures is not supported by a large number of Prolog implementations, but it has several advantages over using the database. It produces less garbage, is thread-safe, reentrant and deals with exceptions without leaking data.

Similar to the `library(assoc)` library, keys can be any Prolog term, but it is not allowed to instantiate or modify a term.

One of the ways to use this library is to generate unique values on backtracking without generating all solutions first, for example to act as a filter between a generator producing many duplicates and an expensive test routine, as outlined below:

    generate_and_test(Solution) :-
            empty_nb_set(Set),
            generate(Solution),
            add_nb_set(Solution, Set, true),
            test(Solution).

empty_nb_set(?Set)
  True if Set is a non-backtrackable empty set.
add_nb_set(+Key, !Set)
  Add Key to Set. If Key is already a member of Set, add_nb_set/3 succeeds without modifying Set.
add_nb_set(+Key, !Set, ?New)
  If Key is not in Set and New is unified to `true`, Key is added to Set. If Key is in Set, New is unified to `false`. It can be used for many purposes:

  `add_nb_set(+, +, false)` | Test membership
  `add_nb_set(+, +, true)` | Succeed only if new member
  `add_nb_set(+, +, Var)` | Succeed, binding Var

gen_nb_set(+Set, -Key)
  Generate all members of Set on backtracking in the standard order of terms. To test membership, use add_nb_set/3.
size_nb_set(+Set, -Size)
  Unify Size with the number of elements in Set.
nb_set_to_list(+Set, -List)
  Unify List with a list of all elements in Set in the standard order of terms (i.e., an ordered list).