XML in a Nutshell, 3rd Edition

Name

function-available( )

Synopsis

                     boolean function-available(string qualifiedName)

function-available() returns true if the argument identifies a function in the processor’s function library; false otherwise. If the qualified name maps to a non-null namespace URI, then it refers to an extension function. Otherwise, it refers to a built-in function from XPath or XSLT. Assuming you’re using a fully conformant processor, however, you don’t need to test for standard functions, only for extension functions.