% examples/reach.dl -- stratified negation + recursion, the CLI demo program.
node(a). node(b). node(c). node(d).
source(a).
edge(a, b). edge(b, c).

reach(X) :- source(X).
reach(X) :- edge(Y, X), reach(Y).

unreachable(X) :- node(X), not reach(X).
sink(X) :- node(X), not edge(X, _).