trying to use abs()

Hello. I'm using Saxon 6.5.3 and am trying to get the abs function to work.
I tried math: and fn: but I'm getting the error: "unknown system function".
I've tried several name spaces, with out any success.

It seems to me that maybe saxon doesn't with it? my xsl namespace is:
http://www.w3.org/1999/XSL/Transform

At this point I'm kinda lost and not really knowing what I need. So how do I
find a libary of math functions? do I have to download some files? do I have
to update the xsl namespace?

Thanks!
LN