Goal:
    x : V.t  s : VSet.t  y : V.t  H1 : x = y  H : False
  ============================
   (x = y \/ VSet.In y s /\ x <> y)


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