Side by side formatting

I am using the side by side formatting example from 
http://www.sagehill.net/docbookxsl/SideBySide.html.

I use this to have section numbers indent left from the body. This 
properly indents the section title to 
provisional-distance-between-starts. However, I was expecting the 
section title to be separated from the label by 
provisional-label-separation if the label is longer than 
provisional-distance-between-starts.

The latter is not happing and the label overwrites part of the section 
title. With that, I am getting the following warnings from fop:
WARNING: Line 1 of a paragraph overflows the available area by 19549mpt. 
(fo:block, location: 3/355188)

Is this a problem with fop (I am using 0.95), or am I misunderstanding 
the side by side formatting behaviour?

Marcel
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