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*).