Extension XPathExpressions in step arguments - 2.7 XPath Extension Functions

I'd like confirmation of my understanding please.

In 2.7 XPath Extension Functions
It lists the extension xpath functions which are required.

Is it true that these are only implemented on explict xpath step options such as "select=" 
but NOT in the step library options.

For example:

p:system-property IS exposed
<p:string-replace match="/doc/episode/@value">
<p:with-option name="replace" select="concat('&quot;',p:system-property('p:episode'), '&quot;')"/>
</p:string-replace>



BUT ... 

p:system-property is NOT exposed (error is thrown)

<p:string-replace match=/doc/episode/@value 
    replace="concat('&quot;',p:system-property('p:episode'), '&quot;')"
</p:string-replace>

-----------------------------------------------------------
David A. Lee
dlee@calldei.com  
http://www.calldei.com
http://www.xmlsh.org