Goal:
  
  G1 : t
  G2 : t
  hsub : full_subgraph G1 G2
  bs := border_set G1 (EdgeSet.diff (E G2) (E G1)) : VSet.t
  _x_ : VSet.elt
  __view_subject__ : VSet.In _x_ bs
  _x1_ : EdgeSet.elt
  _a_ : EdgeSet.In _x1_ (EdgeSet.diff (E G2) (E G1))
  ============================
   (_x_ = _x1_..s \/ _x_ = _x1_..t ->
    VSet.In _x_ (V G1) -> VSet.In _x_ (V G1))


DEBUG:Going to execute:
done
DEBUG LTAC Evaluated term: border_set_spec
DEBUG LTAC Evaluated term: (iffLR
                              (Q:=exists e : EdgeSet.elt,
                                    EdgeSet.In e (EdgeSet.diff (E G2) (E G1)) /\
                                    (_x_ = e..s \/ _x_ = e..t) /\
                                    VSet.In _x_ (V G1))
                              (border_set_spec G1
                                 (EdgeSet.diff (E G2) (E G1)) _x_)
                              _view_subject_)