Pointed out by Wouter S. Hager on the xquery-talk list:

<quote>
Not sure if anyone already noticed, but there's an error in the XPath/XQuery F&O docs, where the example implementation of array:fold will always return an array...
</quote>

It seems that in the "reference implementation" given to specify the function,

then [$zero]

should read

then $zero

to ensure that the function delivers the result implied by the examples and notes, as well as all the test cases in the test suite.

The same error is present in array:fold-left.