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)
  ============================
   (VSet.for_all (fun y : VSet.elt => lsp G1 x y == lsp G2 x y) bs = true)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrapply@0> $1
DEBUG LTAC Evaluated term: VSet.for_all_spec
DEBUG LTAC Evaluated term: H
DEBUG LTAC Evaluated term: (H x _view_subject_)