Goal:
  
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  x3 : V.t
  a : V.t
  H : VSet.In a _z_
  ============================
   (x1 = a \/ VSet.In a _z_)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end