@def x1(u32, string) input output.
@def x2(u32, string) input output.

@def edge(u32, u32) input.
@def reachable(u32, u32) output.

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

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