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


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