The | operator was available in XPath 1.0; the keyword union has been
added in XPath 2.0 as a synonym, because it is familiar to SQL users.
The expression E1 union E2 selects all nodes that are in either E1 or
E2 or both. Both expressions must return sequences of nodes. The results
are returned in document order. For example, /book/(chapter | appendix)/sections returns
all section elements within a chapter or appendix of the
selected book element.
