Goal:
    x : V.t  s : VSet.t  y : V.t
  ============================
   ((VSet.In y (VSet.add x s) -> x = y \/ VSet.In y (VSet.remove x s)) /\
    (x = y \/ VSet.In y (VSet.remove x s) -> VSet.In y (VSet.add x s)))


DEBUG:Going to execute:
repeat match goal with
                  | H:forall a : V.t, _ |- _ => clear H
                  end
Level 1: Backtrace:

No matching clauses for match.