Returns True if the function is in the function library.
The argument must evaluate to a string that is a QName. The QName is expanded into an expanded-name using the namespace declarations in scope for the expression. The function-available function returns True if and only if the expanded-name is the name of a function in the function library. If the expanded-name has a non-null namespace Uniform Resource Identifier (URI), it refers to an extension function; otherwise, it refers to a function defined by XML Path Language (XPath) or XSLT.