xsl:key element is used at the top level of the stylesheet to declare an attribute, or other value,
that may be used as a key to identify nodes using the
key() function within an expression. Each xsl:key
definition declares a named key, which must match the name of the key used in the
The set of nodes to which the key applies is defined by a pattern in the match attribute: for example,
match="ACT|SCENE" then every
ACT element and every
SCENE element is indexed by this key.
The value of the key, for each of these matched elements, is determined by the
This is an expression, which is evaluated for each matched element. If the expression returns a node-set,
the typed value of each node in this node-set acts as a key value. For example, if
use="AUTHOR", then each
AUTHOR child of the matched element supplies one key value.
Keys are not unique: the same value may identify many different nodes
Keys are multi-valued: each matched node may have several (zero or more) values of the key, any one
of which may be used to locate that node
Keys can only be used to identify nodes within a single XML document: the key() function will return nodes
that are in the same document as the current node.
All three attributes,
use, are mandatory.
collation attribute can be used when comparing strings.
Saxon does not yet allow the
xsl:key element to contain a sequence constructor in place