Goal:
  
  G : t
  s0 : VSet.t
  x : V.t
  z : V.t
  e : EdgeOf x z
  s' : VSet.t
  Hs : DisjointAdd x s0 s'
  ============================
   (e.π1 + 0 = e.π1)


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