Comment # 16 on bug 964357 from Ulrich Windl

So I eventually checked it, downgrading the bug priority: If your input file
contains
\begin{lilypond}
whatever...
\end{lilypond\

The lilypond-book parser does not detect an unclosed lilypond environment (not
even a warning!), but passes the lilypond code to LaTeX, which cannot
understand \begin{lilypond} and triggers error "! LaTeX Error: Environment
lilypond undefined.".
Correct input is processed corr4ectly, however.