side effect definition

I just want to make sure that I understand what
the term "side effect free" mean. Is my simplistic
understanding, stated below accurate:

"Assuming no usage of extension functions or elements,
pure XSLT programming is side effect free because it
doesn't maintain global state." -- Gavin

If I do use extension functions or elements, what design
patterns are available to guide me such that I do
*not* violate the "side effect free" feature of XSLT ?

Regards,

Gavin Bong

...........................................................
Be liberal in what you accept, 
and conservative in what you send
- Jon Postel, RFC1122


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list