optional dependencies

[Erik Hatcher]

We have one class that I added (xpath_mapper) that relies on a 3rd  
party gem (libxml).  This particular one can be refactored to fall  
back to REXML if libxml is not installed, as we do with the rest of  
solr-ruby.  However, we have a cool HpricotMapper contribution  
(thanks Jamie!) pending in JIRA: http://issues.apache.org/jira/browse/ 
SOLR-210

How should we incorporate this so as to not negatively impact those  
that don't have Hpricot installed?   In other words, what's the  
recommended Rubyism for this?

Anyone tried out the Mapper stuff besides Jamie and I?  It's become  
my favorite way to push data into Solr.

Thanks,
        Erik