The <index> element

The patternOptimization element may contain one or more index children defining schemes for indexation of template rules. The index element has the following attributes:

Attribute	Applies to	Values	Effect
index.expr	EE	a pattern string	An expression to generate an indexation value from a node under test. Variable values bound during the match of `@pattern` will be in scope for evaluation of this expression.
index.value	EE	a pattern string	An expression to generate an indexation key for the matched template rule. Variable values bound during the match of `@pattern` will be in scope for evaluation of this expression.
mode	EE	xsl:modes	Modes for which this directive is relevant. Default `#all`.
pattern	EE	a pattern string	A pattern to match against a template match pattern!
when	EE	expression	Qualification expression on expression variables from the pattern. Default `true()`.

For details of the @pattern, @when and @mode properties, see Configurable rule optimization.