5.1. Boolean Functions5.1.1. The lang( ) Function5.2. Node-Set Functions5.2.1. The id( ) Function5.3. Number Functions5.3.1. The sum( ) Function5.3.2. The round( ) Function5.4. String Functions5.4.1. The concat( ) Function5.4.2. The normalize-space( ), translate( ), and substring( ) Functions5.4.3. The generate-id( ) Function5.5. Summary