Goal:
  
  s : VSet.t
  s'' : VSet.t
  x : V.t
  hin : VSet.In x s -> False
  H : VSet.In x (VSet.union s s'')
  ============================
   (VSet.In x s'')


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