layout for nested environments

[Gunnar]

Hello.
I would like to use an exercise environment in LyX without using ERT.

The ERT looks like this
\begin{ex}
  What is not Foo?
   \begin{sol}
      Bar is the answer.
   \end{sol}
\end{ex}

So I started to edit the article.layout (code below)
but how can I make  the solution to appear inside the exercise?


------------------ article.layout-----------
Style Exercise
        LatexType       Environment
        LatexName       ex
        ParSep          0.8
        LabelType       Manual
        LabelString     "Uppgift"
        Font
           Shape        Upright
           Family       Sans
        EndFont
        LabelFont
           Series       Bold
           Family       Sans
           Shape        Upright
        EndFont
End

Style Answer
        LatexType       Environment
        LatexName       sol
        ParSep          0.8
        LabelType       Manual
        LabelString     "Svar"
        Font
           Shape        Upright
           Family       Sans
        EndFont
        LabelFont
           Series       Bold
           Family       Sans
           Shape        Upright
        EndFont
End