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# ?Element `@ ?SetVariable

Set membership constraint
*SetVariable*
- A set variable.
*Element*
- A ground term or a variable.

## Description

Constrain SetVariable to include Element.
If Element is a variable then if SetVariable is a ground singleton,
then Element is unified with its single element, otherwise the constraint
is suspended until Element or SetVariable is ground.

### Fail Conditions

Fails if Element can not be a member of SetVariable.
### Resatisfiable

No.
## Examples

?- S `:: []..[a,b], a `@ S, glb_poss(S,G,P).
G = [a], P = [b]
?- S `:: []+[a,b], c `@ S.
no
?- S `:: [a]+[b,c], a `@ S, glb_poss(S,G,P).
G = [a], P = [b,c]
?- S `:: []..[a,b], X `@ S, glb_poss(S,G,P).
G = [], P = [a,b]
?- S `:: []..[a,b], X `@ S, X=b, glb_poss(S,G,P).
G = [b], P = [a]
?- S `:: [a]+[b,c]:C, X `@ S, C=1.
X = a

## See Also

in / 2, `-@ / 2, notin / 2, `:: / 2