Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  a : V.t  H : x2 = a
  ============================
   (x1 = a \/ x2 = a \/ VSet.In a _z_)


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