SINGLE-VALUED-SLOT is a constraint on the second argument of a binary relation that is conditional on the first argument to the relation being an instance of a given class. It is like unary-function, except it is local to the values of the relation on instances of the given subset of the domain.
(<=> (Single-Valued-Slot ?Class ?Binary-Relation)
(= (Slot-Cardinality ?Class ?Binary-Relation) 1))