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Standard excerpt:The use attribute is an expression specifying the values of the key; the expression is evaluated once for each node that matches
the pattern. If the result is a node-set, then for each node in the node-set, the node that matches the pattern has a key
of the specified name whose value is the string-value of the node in the node-set; otherwise, the result is converted to a
string, and the node that matches the pattern has a key of the specified name with value equal to that string. It is an error
for the value of the use attribute to contain a VariableReference.
Examples (xslt:xml): Overview
Interactive xlab: [xsl:key]