**Defined in theory: Slot-constraint-sugar****Source code: slot-constraint-sugar.lisp**

**Documentation:**A domain instance i CAN-HAVE-ONE value for a slot R if there is at most 1 value v for which R(i,v) holds.

Asserting (CAN-HAVE-ONE ?i R) in the definition of some class C, where ?i is the instance variable for that class, is another way of specifying that C is a domain restriction of R and R is a single-valued slot on C.

**Arity:**2

(<=> (Can-Have-One ?Instance ?Binary-Relation) (Has-At-Most ?Instance ?Binary-Relation 1))

**See-Also:**has-at-most single-valued-slot domain