Goal:
  
  s : VSet.t
  s'' : VSet.t
  x : V.t
  hin : ~ VSet.In x s
  H : VSet.In x s \/ VSet.In x s''
  ============================
   (VSet.In x s'')


DEBUG:Going to execute:
match goal with
           | H:context [ ?x <> ?y ]
             |- _ => assert new (x = y \/ x <> y) by
             apply VSetDecide.MSetDecideAuxiliary.dec_eq
           | H:context [ ~ VSet.In ?x ?s ]
             |- _ => assert new (VSet.In x s \/ ~ VSet.In x s) by
             apply VSetDecide.MSetDecideAuxiliary.dec_In
           | |- context [ ?x <> ?y ] => assert new 
             (x = y \/ x <> y) by apply VSetDecide.MSetDecideAuxiliary.dec_eq
           | |- context [ ~ VSet.In ?x ?s ] => assert new
             (VSet.In x s \/ ~ VSet.In x s) by
             apply VSetDecide.MSetDecideAuxiliary.dec_In
           end