Goal:
    G1 : t  ext : EdgeSet.t  x : VSet.elt
  ============================
   (VSet.In x border_set <->
    (exists e : EdgeSet.elt,
       In e (EdgeSet.elements ext) /\
       (x = e..s \/ x = e..t) /\ VSet.In x (V G1)))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2