A Chonáin,This was recently discussed on the IRTRAD-L list. Here's the URL for the explanatory message there:
http://listserv.heanet.ie/cgi-bin/wa?A2=ind9911&L=irtrad-l&F=&S=&P=29552
I hope that works, I don't know how to do a 'blue clicky thingy.'
-- Áine