DOCBOOK: Linenumbering in literallayout

I have a document with some programlistings, which are discussed line-by-line
by various sections. Everywhere I describe a line, the original line
reappears as an excerpt. However, the only way to number them in DocBook seems
to be to start by line 1, while I'd like to be able to use some sort of
attribute that a programlisting starts at line 4, for example.

Am I overseeing already built-in semantics which could do the job or should
I start customizing?

Thanks for your time,

 - Rowan van der Molen

-- 
Don't fear fear.