[nem-en] recursive types

[Wiktor Zychla]

>    | Var( name ) =>
>     when ( !a.Contains( name ) )
>      a.Add( name );

I've found that cosuming the value of Add helps the compiler to accept the 
code:

    | Var( name ) =>
     when ( !a.Contains( name ) )
        mutable dummy = 0;
        dummy = a.Add( name );
}

But this is awful! I still look for another solution.

Interestingly, following does not compile

    | Var( name ) =>
     when ( !a.Contains( name ) )
        mutable dummy = a.Add( name );
}

with "let binding not followed by any expression".

Wiktor