This is a newer function that has not the right signature. Since the arguments 
are often calculated, we should allow empty:

11.2.6 fn:in-scope-prefixes
fn:in-scope-prefixes($element as element()?) as xs:string*
Summary: Returns the prefixes of the in-scope namespaces for $element. If 
$element is the empty sequence, it returns the empty sequence. For namespaces 
that have a prefix, it returns the prefix as an xs:NCName. For the default 
namespace, which has no prefix, it returns the zero-length string.