\documentclass{article}

\input{include/def.tex}

\begin{document}

If the include file in the preamble is not available,
the automatic line width detection would fail and the
snippet below would have only one bar per line.

\begin{lilypond}
\relative c' {
  a d a c e d a c
  f g d e a d a c
}
\end{lilypond}

Brought to you by \signature.

\end{document}