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The domains of variables are internally represented compactly as
*FD set* terms. The details of this representation are subject to
change and should not be relied on. Therefore, a number of operations
on FD sets are provided, as such terms play an important role in the
interface. **Under no circumstances should you try to construct
FD sets with other operations such as unification.** The following
operations are the primitive ones:

`is_fdset(`

`+Set`)-
`Set`is a valid FD set. `empty_fdset(`

`?Set`)-
`Set`is the empty FD set. `fdset_parts(`

`?Set`,`?Min`,`?Max`,`?Rest`)-
`Set`is an FD set, which is a union of the non-empty interval`[Min,Max]`and the FD set`Rest`, and all elements of`Rest`are greater than`Max`+1.`Min`and`Max`are both integers or the atoms`inf`

and`sup`

, denoting minus and plus infinity, respectively. Either`Set`or all the other arguments must be ground.

The following operations can all be defined in terms of the primitive ones, but in most cases, a more efficient implementation is used:

`empty_interval(`

`+Min`,`+Max`)-
`[Min,Max]`is an empty interval. `fdset_interval(`

`?Set`,`?Min`,`?Max`)-
`Set`is an FD set, which is the non-empty interval`[Min,Max]`. `fdset_singleton(`

`?Set`,`?Elt`)-
`Set`is an FD set containing`Elt`only. At least one of the arguments must be ground. `fdset_min(`

`+Set`,`-Min`)-
`Min`is the lower bound of`Set`. `fdset_max(`

`+Set`,`-Min`)-
`Max`is the upper bound of`Set`. This operation is linear in the number of intervals of`Set`. `fdset_size(`

`+Set`,`-Size`)-
`Size`is the cardinality of`Set`, represented as`sup`

if`Set`is infinite. This operation is linear in the number of intervals of`Set`. `list_to_fdset(`

`+List`,`-Set`)-
`Set`is the FD set containing the elements of`List`. Slightly more efficient if`List`is ordered. `fdset_to_list(`

`+Set`,`-List`)-
`List`is an ordered list of the elements of`Set`, which must be finite. `range_to_fdset(`

`+Range`,`-Set`)-
`Set`is the FD set containing the elements of the`ConstantRange`(see Syntax of Indexicals)`Range`. `fdset_to_range(`

`+Set`,`-Range`)-
`Range`is a constant interval, a singleton constant set, or a union of such, denoting the same set as`Set`. `fdset_add_element(`

`+Set1`,`+Elt``-Set2`)-
`Set2`is`Set1`with`Elt`inserted in it. `fdset_del_element(`

`+Set1`,`+Elt`,`-Set2`)-
`Set2`is like`Set1`but with`Elt`removed. `fdset_disjoint(`

`+Set1`,`+Set2`)-
The two FD sets have no elements in common.

`fdset_intersect(`

`+Set1`,`+Set2`)-
The two FD sets have at least one element in common.

`fdset_intersection(`

`+Set1`,`+Set2`,`-Intersection`)-
`Intersection`is the intersection between`Set1`and`Set2`. `fdset_intersection(`

`+Sets`,`-Intersection`)`Intersection`is the intersection of all the sets in`Sets`.`fdset_member(`

`?Elt`,`+Set`)-
is true when

`Elt`is a member of`Set`. If`Elt`is unbound, then`Set`must be finite. `fdset_eq(`

`+Set1`,`+Set2`)-
Is true when the two arguments represent the same set i.e. they are identical.

`fdset_subset(`

`+Set1`,`+Set2`)-
Every element of

`Set1`appears in`Set2`. `fdset_subtract(`

`+Set1`,`+Set2`,`-Difference`)-
`Difference`contains all and only the elements of`Set1`that are not also in`Set2`. `fdset_union(`

`+Set1`,`+Set2`,`-Union`)-
`Union`is the union of`Set1`and`Set2`. `fdset_union(`

`+Sets`,`-Union`)`Union`is the union of all the sets in`Sets`.`fdset_complement(`

`+Set`,`-Complement`)-
`Complement`is the complement of`Set`wrt.`inf..sup`

.

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