Goal:
    G1 : t  ext : EdgeSet.t  x : VSet.elt  e : Edge.t  s : VSet.t
  ============================
   (VSet.In x (add_from_orig e..s (add_from_orig e..t s)) <->
    (x = e..s \/ x = e..t) /\ VSet.In x (V G1) \/ VSet.In x s)


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