Relation Has-One

Instance-Of: Binary-Relation, Relation, Set
Range: Binary-Relation
Arity: 2
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.

Equivalence Axioms for Has-One:

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

Axioms for Has-One:

(Binary-Relation ?Binary-Relation)