> What is the base type of (), is it a node or an atomic, or something else?

Nevermind, it explains it here:

http://www.w3.org/TR/xpath20/#doc-xpath-SequenceType

"With the exception of the special type empty-sequence(), a sequence
type consists of an item type that constrains the type of each item in
the sequence, and a cardinality that constrains the number of items in
the sequence. Apart from the item type item(), which permits any kind
of item, item types divide into node types (such as element()) and
atomic types (such as xs:integer)."


-- 
Andrew Welch
http://andrewjwelch.com