Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  a : V.t
  ============================
   ((VSet.In a (VSet.add x0 (VSet.add x1 (VSet.add x2 _z_))) ->
     VSet.In a (VSet.add x1 (VSet.add x2 (VSet.add x0 _z_)))) /\
    (VSet.In a (VSet.add x1 (VSet.add x2 (VSet.add x0 _z_))) ->
     VSet.In a (VSet.add x0 (VSet.add x1 (VSet.add x2 _z_)))))


DEBUG:Going to execute:
match goal with
           | H:forall a : V.t, _, _:context [ VSet.In ?x _ ]
             |- _ => let P := type of (H x) in
                     assert
                     new
                     P
                     by
                     exact
                     (H x)
           | H:forall a : V.t, _
             |- context [ VSet.In ?x _ ] =>
                 let P := type of (H x) in
                 assert
                 new
                 P
                 by
                 exact
                 (H x)
           | H:forall a : V.t, _, _:context [ ?x = _ ]
             |- _ => let P := type of (H x) in
                     assert
                     new
                     P
                     by
                     exact
                     (H x)
           | H:forall a : V.t, _
             |- context [ ?x = _ ] =>
                 let P := type of (H x) in
                 assert
                 new
                 P
                 by
                 exact
                 (H x)
           | H:forall a : V.t, _, _:context [ _ = ?x ]
             |- _ => let P := type of (H x) in
                     assert
                     new
                     P
                     by
                     exact
                     (H x)
           | H:forall a : V.t, _
             |- context [ _ = ?x ] =>
                 let P := type of (H x) in
                 assert
                 new
                 P
                 by
                 exact
                 (H x)
           end