parentOf(adam, able).
parentOf(eve, able).
parentOf(adam, cain).
parentOf(eve, cain).
male(adam).
female(eve).

motherOf(X,Y) :- parentOf(X,Y), female(X).
fatherOf(X,Y) :- parentOf(X,Y), male(X).
siblings(X,Y) :- parentOf(X,P1), parentOf(Y,P1), X \= Y.

isParent(X) :- parentOf(X,_).

childLess(X) :- \+ isParent(X).