Goal:
    _z_ : VSet.t  a : V.t
  ============================
   (VSet.In a _z_ <-> VSet.In a _z_)


DEBUG:Going to execute:
progress change T with V.t
DEBUG LTAC Evaluated term: VSet.elt
Level 2: Backtrace:

Failed to progress.
Level 2: Backtrace:

Failed to progress.