Alternate stylesheet based on availability

Hi all,



I need to know if  can fill a very specific need for me. I am
relatively unfamiliar with XSL so forgive the generality of this
message.



I would like to use a local stylesheet.xsl file (i.e. in the same
directory) unless it doesn't exist and then I would like to use one on a
different server entirely.



I haven't been able to find the logic necessary to make this happen. Any
suggestions would be most welcome.



Thanks,



Matt