Goal:
  
  G1 : t
  G2 : t
  hsub : full_subgraph G1 G2
  bs := border_set G1 (EdgeSet.diff (E G2) (E G1)) : VSet.t
  ============================
   (forall x : VSet.elt, VSet.In x bs -> VSet.In x (V G1))


DEBUG:Going to execute:
move  =>? /border_set_spec [? [_ [//]]]
DEBUG LTAC Evaluated term: (border_set ?G1 ?ext)
DEBUG LTAC Evaluated term: (forall x : VSet.elt,
                            VSet.In x bs -> VSet.In x (V G1))