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


DEBUG:Going to execute:
subst ++
DEBUG LTAC Evaluated term: eq
Level 1: Backtrace:

Failed to progress.
Level 1: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.