Goal:
    s' : VSet.t  t0 : VSet.t  a : V.t  H : VSet.In a s'
  ============================
   (VSet.In a (VSet.union t0 s'))


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.