Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  a : V.t
  ============================
   (VSet.In a (VSet.add x0 (VSet.add x1 _z_)) <->
    VSet.In a (VSet.add x1 (VSet.add x0 _z_)))


DEBUG:Going to execute:
unfold not, iff;
            repeat
             match goal with
             | |- context [ True -> False ] =>
                   rewrite -> Decidable.not_true_iff
             | |- context [ False -> False ] =>
                   rewrite -> Decidable.not_false_iff
             | |- context [ (?P -> False) -> False ] =>
                   rewrite -> (Decidable.not_not_iff P) by dec
             | |- context [ (?P -> False) -> ?Q -> False ] =>
                   rewrite -> (Decidable.contrapositive P Q) by dec
             | |- context [ (?P -> False) \/ ?Q ] =>
                   VSetDecide.MSetLogicalFacts.or_not_l_iff P Q dec
             | |- context [ ?P \/ (?Q -> False) ] =>
                   VSetDecide.MSetLogicalFacts.or_not_r_iff P Q dec
             | |- context [ (?P -> False) -> ?Q ] =>
                   rewrite -> (Decidable.imp_not_l P Q) by dec
             | |- context [ (?P -> False) /\ (?Q -> False) ] =>
                   rewrite <- (Decidable.not_or_iff P Q)
             | |- context [ ?P -> ?Q -> False ] =>
                   rewrite <- (Decidable.not_and_iff P Q)
             | |- context [ ?P /\ (?Q -> False) ] =>
                   rewrite <- (Decidable.not_imp_iff P Q) by dec
             | |- context [ (?Q -> False) /\ ?P ] =>
                   rewrite <- (Decidable.not_imp_rev_iff P Q) by dec
             end