Goal:
    G : t  HI : invariants G  HG : acyclic_no_loop G  n : Z  x : V.t  y : V.t
  ============================
   (leq_vertices G n x y <->
    (if VSet.mem y (V G)
     then (Some n <= lsp G x y)%nbar
     else n <= 0 /\ (x = y \/ (Some n <= lsp G x (s G))%nbar)))


DEBUG:Going to execute:
fun x => generalize eq_refl; pattern x at - 1; case x