Goal:
  
  x : V.t
  s : VSet.t
  s' : VSet.t
  H : VSetProp.Add x s s'
  z : VSet.elt
  Hz : VSet.In z s
  ============================
   (x = z)


DEBUG:Going to execute:
match goal with
           | |- _ -> _ => intro; t_tauto_intuit
           | |- _ => t_reduce; t_solver
           end