lawyer::person.
person[hasChild => person, marriedTo => person].

john:lawyer.
john[hasChild -> mary].

// simple rule
?x:parent :- ?x:person, ?x[hasChild -> ?y].

// simple rule with non-monotonic negation
//  from this follows: john:singleParent
?x:marriedParent :- ?x:parent, ?x[marriedTo -> ?y].
?x:singleParent :- ?x:parent, not ?x:marriedParent.

// uncomment this fact to marry john and make him not single anymore

// john[marriedTo -> jill].



// (inferring) semantics of attribute signature
//  uncomment to get the additional inference mary:person.

// ?y:?z :- ?x:?v[?w -> ?y], ?v[?w => ?z].