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<h1>Union</h1>
<p>The <code>|</code> operator was available in XPath 1.0; the keyword <code>union</code> has been
added in XPath 2.0 as a synonym, because it is familiar to SQL users.</p>
<p>The expression <code>E1 union E2</code> selects all nodes that are in either <code>E1</code> or
<code>E2</code> or both. Both expressions must return sequences of nodes. The results
are returned in document order. For example, <code>/book/(chapter | appendix)/sections</code> returns
all <code>section</code> elements within a <code>chapter</code> or <code>appendix</code> of the
selected <code>book</code> element.</p>
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