Keys

• Key instruction and key function.
• Key matching based on element context
• Keys usage for cross-referencing
• key as the first step of an XPath expression
• Key element contents instead of use attribute
• Several keys elements of the same name
• The same node matched by several keys
• Sequence of several values as the second argument of key function
• Optional third argument of key function
• Using keys in variables or parameters
• A useful example of context change with the third argument of key
• Function key inside an XPath expression