% UBC CPSC 312
% A small example to illustrate Prolog's tracing.
% Credit: David Poole

:- dynamic f/0. % needed b/c f is otherwise undefined.

a :- b, c, d.
a :- e.
b.
c :- s.
c :- t.
d :- f.
e :- t.
e :- s.
s.
t.