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


DEBUG:Going to execute:
match goal with
           | x:_ |- _ => subst x
           end
Level 1: Backtrace:

Cannot find any non-recursive equality over x.
Level 1: Backtrace:

Cannot find any non-recursive equality over x.
Level 0: Backtrace:

Cannot find any non-recursive equality over x.