Specify the type of an item.
The following construct(s) refer to this construct:
An ItemType specifies the type of an XQuery item by using
one of:
a qualified name that is interpreted as an atomic type,
a KindTest matching processing instruction nodes, comment
nodes, text nodes or any kind of nodes,
any single item.
The item type xs:decimal is a QName matching the value
47.11 since it is of the atomic type xs:decimal.
document-node(element(book)) matches a document node
containing exactly one element node that is matched by the ElementTest
element(book).
item() matches for example the atomic value
42 or the element <bib>.
