Go to standardXSLT function: boolean element-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 element-available function returns true if and only if the expanded-name is the name of an
instruction. If the expanded-name has a namespace URI equal to the XSLT namespace URI, then it refers to an element defined
by XSLT. Otherwise, it refers to an extension element. If the expanded-name has a null namespace URI, the element-available
function will return false.
Examples (xslt:xml): Overview
62:2
Interactive xlab: [xsl:value-of] [xsl:template]