Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  x : V.t
  y : V.t
  n : Z
  p : SPath (V G) x y
  Hp : sweight p = n
  z : Z
  pi : SPath (V G) y x
  Hpi : sweight pi = z
  h : V.t
  loop : SPath (VSet.union (V G) (nodes (to_pathOf pi))) h h
  H : 0 < sweight loop
  ltb : 0 < z + n
  ============================
   (sweight loop <= 0 -> z <= - n)


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op;
            Zify.zify_iter_specs