Many staff contexts do not nest

[Richard Shann]

The documentation says that staff groups can be nested, but many don't
appear to work, e.g. this

\version "2.18.0"
\score {
        <<
             \new GrandStaff <<
                                \new Staff { s1*4/4 }
                                 \new ChoirStaff <<
                                                    \new Staff
{ s1*4/4 }
                                                    \new Staff
{ s1*4/4 }
                                                    \new Staff
{ s1*4/4 }
                                                >>
                             >>
        >>
    }

has no GrandStaff. 
Is something missing at
http://www.lilypond.org/doc/v2.18/Documentation/notation/displaying-staves
and can anyone explain the rules (I am trying to write a staff braces
checker ...)

Richard Shann