Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  x : VSet.elt
  Hx : VSet.In x (V G)
  n : Z
  H : lsp (s G) x = Some n
  ============================
   (exists n0 : Z, Some n = Some n0 /\ 0 <= n0)


DEBUG:Going to execute:
eexists