p:wrap's group-adjacent

The p:wrap step has a group-adjacent option.  I understand what p:wrap does,
and I think I understand what group-adjacent is trying to do.  But I can¹t
get it to work.

I think I could get it working if someone would be so kind as to translate
the middle sentence in this paragraph from the WD into English:

The group-adjacent option can be used to group adjacent matching nodes in a
single wrapper element. The specified XPath expression is evaluated for each
matching node with that node as the XPath context node. Whenever two or more
adjacent matching nodes have the same ³group adjacent² value, they are
wrapped together in a single wrapper element.

I appreciate any help,

James Garriss
http://garriss.blogspot.com