Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  a : V.t  H0 : x1 = a
  ============================
   (x1 = a \/ VSet.In a _z_)


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]