zero-or-one

The zero-or-one() function returns its argument unchanged, provided that it is a sequence containing no more than one item. In other cases, it reports an error.

Argument	Data Type	Meaning
value	item()*	The input value. Although the function signature says that any sequence of items is allowed, a runtime error will occur if the number of items is more than one
Result	Item()	The same as the supplied value, after checking to ensure that it is either an empty sequence or contains a single item