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


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro