Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  x : VSet.elt
  Hx : VSet.In x (V G)
  ============================
   (forall z : Z,
    lsp (s G) x = Some z -> exists n : Z, Some z = Some n /\ 0 <= n)


DEBUG:Going to execute:
intros n H; eexists