prolog-recursion-3.pro

% testing the first clause
ancestor(zeb, john_boy_sr). % yes
ancestor(john_boy_sr, zeb). % no

% testing the second clause
ancestor(zeb, john_boy_jr). % yes

% let's ask, "Who is an ancestor of zeb"
ancestor(zeb, Who).         % john_boy_sr ; john_boy_jr
% "Who is john_boy_jr an ancestor
ancestor(Who, john_boy_jr). % john_boy_sr ; zeb