Go to standardXSLT function: boolean function-available (string)
Standard excerpt:
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 URI, then it refers to an extension function;
otherwise, it refers to a function defined by XPath or XSLT.
Examples (xslt:xml): Overview
63:2
Interactive xlab: [xsl:value-of] [xsl:template]