Variable weirdness

[David Sumbler]

I had the following lines in the main file of my current Lilypond
project:

\book {
  \bookOutputName "../firstCello"
    partName = "Cello 1"
    \include "frontcover.ily"
    \bookpart { %music...

The file "frontcover.ily" contains a \bookpart block which prints a
front cover with title, composer etc. - these are defined elsewhere.
 But it needs one more variable, viz. 'partName'.

I discovered that Lilypond will not accept a variable definition in
Lilypond format in the position I have put it: at the top level of a
\book block.  Nor will it accept it in a \bookpart block.  But at a
higher or a lower level, it will.

This seems a bit weird (to say the least), in view of the fact that by
replacing the line 
        partName = "Cello 1"
with the Scheme form
        #(define partName "Cello 1")
everything works as intended.

Is there any useful reason why a variable cannot be defined in Lilypond
format in these contexts?

David