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


DEBUG:Going to execute:
repeat
            match goal with
            | H:?T
              |- _ =>
                  progress change T with V.t in H;
                   repeat
                    match goal with
                    | J:_ |- _ => progress change T with V.t in J
                    | |- _ => progress change T with V.t
                    end
            | H:forall x : ?T, _
              |- _ =>
                  progress change T with V.t in H;
                   repeat
                    match goal with
                    | J:_ |- _ => progress change T with V.t in J
                    | |- _ => progress change T with V.t
                    end
            end