Overlapping trees in XML
xmlhack: One tree isn’t enough talks about a couple of proposals for encoding overlapping trees.
LMNL is a non XML markup language which allows for overlapping ranges.JITT (Just in time trees) is an XML based system which can derive different trees by parsing the file differntly. They have a page on Overlapping Hierarchies/Concurrent Markup.
This looks like it deserves some more attention.