Returns $arg if it contains exactly one item. Otherwise, raises an error
[err:FORG0005].
For detailed type semantics, see
Section 7.2.16 The fn:zero-or-one,
fn:one-or-more, and fn:exactly-one functions[FS].
The sequence of items.
If you pass in a single value,
it is treated as a ValueIterator with that single item; therefore, if you
pass in an array, the array is treated as a single value (not as one value
for each item in the array). If you mean to pass in
the values of each item in the array, then you can call
xdmp.arrayValues on the array.
argValueIteratorString
fn.exactlyOne(["hello"])
=> ["hello"]
fn.exactlyOne(xdmp.arrayValues(["hello", "goodbye"]))
=> XDMP-NOTONEITEM exception (because there are 2 items)