Goal:
    x : V.t  s : VSet.t
  ============================
   (~ VSet.In x (VSet.remove x s))


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.