\bookpart mystery

[David Sumbler]

In my efforts (largely successful) to find a way easily to control
conditional compilation, I came across this apparent anomaly.

The following two short examples work as one might expect:

%%%%%
\book {
  \score { g }
}
%%%%%

%%%%%
var =
\book {
  \score { g }
}

\var
%%%%%

Adding a \bookpart to the first also works as expected:

%%%%%
\book {
  \bookpart {
    \score { g }
  }
}
%%%%%

But putting this into a variable thus:

%%%%%
var =
\book {
  \bookpart {
    \score { g }
  }
}

\var
%%%%%

causes parsing errors.

For my better understanding of how LilyPond works, can somebody explain
to me why this happens?

David