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)
  ============================
   (forall (x y : V.t) (e : EdgeOf G1 x y),
    (subgraph_on_edge hsub x y e).π1 >= e.π1)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2