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

**Documentation:**Binary relation R HAS-ONE value on domain instance i if there exists exactly one value v such that R(i,v) holds.

When used in the definition of a class where ?i is the instance variable, (HAS-ONE ?i R) means that the slot R must always have a value, and only one value, when applied to instance of that class.

**Arity:**2**Range:**Binary-relation

(<=> (Has-One ?Instance ?Binary-Relation) (And (Binary-Relation ?Binary-Relation) (= (Value-Cardinality ?Instance ?Binary-Relation) 1)))