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:
f x
Level 3: evaluation returns
<coq-core.plugins.tauto::with_uniform_flags@0>
where
f := fun flags => <Coq.Init.Tauto.intuition_gen> flags tac
     where
     tac := progress VSetDecide.MSetDecideAuxiliary.substMSet; intuition
             VSetDecide.MSetDecideAuxiliary.fsetdec_rec
     of type
     tacvalue
of type
tacvalue of type tacvalue

Level 2: evaluation returns
<coq-core.plugins.tauto::with_uniform_flags@0>
where
f := fun flags => <Coq.Init.Tauto.intuition_gen> flags tac
     where
     tac := progress VSetDecide.MSetDecideAuxiliary.substMSet; intuition
             VSetDecide.MSetDecideAuxiliary.fsetdec_rec
     of type
     tacvalue
of type
tacvalue of type tacvalue