Goal:
  
  G1 : t
  G2 : t
  hsub : full_subgraph G1 G2
  bs := border_set G1 (EdgeSet.diff (E G2) (E G1)) : VSet.t
  H : forall x : VSet.elt, VSet.In x bs -> VSet.In x (V G1)
  x : VSet.elt
  hx : VSet.In x (V G1)
  y : VSet.elt
  hy : VSet.In y (V G1)
  _x_ : V.t
  _y_ : V.t
  _x1_ : Z
  _p_ : EdgeSet.In (_x_, _x1_, _y_) (E G1)
  ============================
   (_x1_ >= _x1_)


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