-- so I wouldn't expect to be able to use the same definition in
multiple match attributes without defining it each time, which isn't
helpful on the "define in one place, debug, use with confidence"
front.

-- Graydon

Oh but that's XSLT use rather than pure XPath so you can do that already
in XSLT 2 define the function using xsl:function and use it in any XPath
within the stylesheet.

XPath3 gives a way of defining functions within XPath itself, which is
useful especially if ou are using XPath directly not via XSLT or XSquery