error

The error() function can be called when the application detects an error condition; it causes evaluation of the XPath expression as a whole to fail.

Argument	Data Type	Meaning
value (optional)	item()?	The value can be used to identify the error, in an implementation-defined way. If the value is a string, it will probably be used as an error message
Result	None	This function does not return a result; it always raises an error