Goal:
    x : VSet.elt  p : VSet.t  H : ~ VSet.In x p
  ============================
   (VSet.Equal (VSet.remove x (VSet.add x p)) p)


DEBUG:Going to execute:
unfold VSet.Empty, VSet.Subset, VSet.Equal in *
Level 1: Backtrace:

x is used in conclusion.
Level 1: Backtrace:

x is used in conclusion.
Level 1: Backtrace:

x is used in conclusion.
Level 0: Backtrace:

x is used in conclusion.