% file:    ancestor.pro
% author:  Robert Keller
% purpose: Logic rules for ancestry

% Here a graph is represented as a list of ordered pairs
% of the form [Parent, Child]

% One rule for a Parent having a Child

child(Parent, Child, Graph) :- 
    member([Parent, Child], Graph). 

% Two rules for an Ancestor having a Descendant

% A child is a descendant.

isDescendant(Ancestor, Desc, Graph) :- 
    child(Ancestor, Desc, Graph). 

% A descendant of a child is a descendant

isDescendant(Ancestor, Desc, Graph) :- 
    child(Ancestor, Child, Graph), 
    isDescendant(Child, Desc, Graph). 

test :- isDescendant(e, a, [[a,b], [b, c], [c,d], [b,e], [a,f]]).