Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  v : VSet.elt
  s : VSet.t
  E : VSet.mem v (V G1) = false
  H : VSet.In x s
  ============================
   (x = v /\ VSet.In x (V G1) \/ VSet.In x s)


DEBUG:Going to execute:
match reverse goal with
           | |- ?X1 => <Coq.Init.Tauto.is_unit_or_eq> flags X1; constructor 1
           | H:?X1 |- _ => <Coq.Init.Tauto.is_empty> flags X1; elim H
           | _ => Ltac.assumption
           end