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:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2