Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  y : V.t
  n0 : (Some n <= lsp G x (s G))%nbar -> False
  ============================
   (false = true <-> (Some n <= lsp G x (s G))%nbar)


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]