Goal:
    x : VSet.elt  p : VSet.t
  ============================
   (VSet.Equal (VSet.add x (VSet.add x p)) (VSet.add x p))


DEBUG:Going to execute:
progress decompose record H; clear H
Level 1: Backtrace:

Failed to progress.
Level 1: Backtrace:

Failed to progress.
Level 1: Backtrace:

Failed to progress.
Level 1: Backtrace:

Failed to progress.