[plt-scheme] extending plt-match?

The docs say that the plt-match.ss library "uses keywords for each of 
the different pattern matches, making the syntax both extensible and 
more clear." I'm curious what "extensible" means here. Is there some way 
to invent new forms recognized by the match form?

Specifically, I'm interested in inventing new struct-like forms that I 
can extend the match construct to recognize. Is there a good way to do this?

Thanks,
Dave