general string replace?

I am having some trouble using Jeni Tennison's general
string replace template that is described in the FAQ:
http://www.dpawson.co.uk/xsl/stringreplace.html

Specifically, when using XT I get the error message
"character not allowed."  When using Xalan, I get the
more helpful message: 
XSLT Error
(javax.xml.transform.TransformerConfigurationException):
The value of attribute "test" must not contain the '<'
character.

It is referring to this line in the replace_strings
template:

<xsl:when test="$search < count(document
('')//foo:search)">

Is the problem that this template only works with
certain XSLT processors (I think Saxon was mentioned)?
 Or is something else going on?

It seems like a cool template; I'd like to use it if I
can figure out how.

Chris


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