Goal:
    G : t  x : V.t  y : V.t  p : PathOf G x y  q : PathOf G y x
  ============================
   (weight p + weight q <= 0 -> weight p <= - weight q)


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