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# intersection(+List1, +List2, ?Common)

Succeeds if Common unifies with the list which contains the common elements
of List1 and List2.
*+List1*
- List.
*+List2*
- List.
*?Common*
- List or variable.

## Description

Common is unified with a list which contains the common elements of
List1 and List2.
The definition of this Prolog library predicate is:

intersection([], _, []).
intersection([Head|L1tail], L2, L3) :-
memberchk(Head, L2),
!,
L3 = [Head|L3tail],
intersection(L1tail, L2, L3tail).
intersection([_|L1tail], L2, L3) :-
intersection(L1tail, L2, L3).

This predicate does not perform any type testing functions.
This predicate works properly for set operations only, so repeated
elements and variable elements should not be used in the lists.

### Modes and Determinism

- intersection(+, +, -) is det

### Fail Conditions

Fails if Common does not unify with the list which contains the common
elements of List1 and List2.
### Resatisfiable

Yes.
## Examples

Success:
intersection([1,2],[2,3],L). (gives L=[2]).
intersection([a,d],[a,b,c],[a]).
Fail:
intersection([a,b],[a,b],[b]).

## See Also

subtract / 3, memberchk / 2, union / 3