Goal:
  
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  x3 : V.t
  a : V.t
  H0 : x2 = a
  ============================
   (x1 = a)


DEBUG:Going to execute:
rewrite -> H in *; clear H