Ancestor

The ancestor relationship.

Source: The Art of Prolog

Program source code: ancestor.pl

/*
	ancestor(Ancestor,Descendant)  :-
		Ancestor is an ancestor of Descendant.
*/
	ancestor(Ancestor,Descendant) :-
		parent(Ancestor,Descendant).
	ancestor(Ancestor,Descendant) :-
		parent(Ancestor,Person), ancestor(Person,Descendant).
 
%	Program 2.5: The ancestor relationship