Bug in Lilypond

I have the following script:

\version "2.26.0"
\book {
 =C2=A0 =C2=A0 \bookpart{
 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0foo =3D { c'1 }=C2=A0 =C2=A0 =C2=A0 <--=
 Fehler: syntax error, unexpected SYMBOL.
 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0\score {
 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0\new Staff \foo
 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0}
 =C2=A0 =C2=A0 =C2=A0}%%bookpart
}%%book

The definition of foo is not allowed within bookpart.

Consequences: within bookpart, lilypond-definitions must be replaced by=20
scheme-definitions.

In the description of lilypond often is recommended to use=20
lilypond-variables, to prepare songs.
I expect, lilypond definitions of variables like "foo =3D { c'1}" should=
=20
be accepted.


your sincerely

Michael Jakob