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:
trivial