.decl edge(from:symbol, to:symbol)
.decl reachable(from:symbol, to:symbol)
.decl on_cycle(node:symbol)

edge("a", "b").
edge("b", "c").
edge("c", "d").
edge("d", "b").
edge("e", "f").

reachable(x, y) :- edge(x, y).
reachable(x, z) :- reachable(x, y), edge(y, z).

on_cycle(x) :- reachable(x, x).

.output reachable
.output on_cycle