XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (302 page)

BOOK: XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition
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to process the four selected elements in the order indicated.

If there is no contained expression, that is, if the
ParenthesizedExpr
is written as
()
, then its value is the empty sequence. For example, the expression
$a
union
()
returns the union of the sequence
$a
and the empty sequence; this has the effect of returning all the nodes in
$a
, in document order and with duplicates removed. The
union
operator (which can also be written
|
) is described in Chapter 9.

One special case where the meaning of parentheses may not be immediately apparent is in conjunction with predicates. Predicates are used to filter the items in a sequence; for example,
$seq[.>= 0]
selects all the items in the sequence
$seq
whose value is greater than zero. As explained in Chapter 9, the meaning of a predicate is subtly different when it appears as part of an axis step. The result of this distinction is that:

  • ancestor::node()[1]
    selects the innermost ancestor of the context node (that is, its parent)
  • (ancestor::node())[1]
    selects the outermost ancestor of the context node (that is, the root of the tree).

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