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


DEBUG:Going to execute:
repeat
            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