Everyone makes mistakes in their syntax once in a while. Why wait for the service restart to discover that your index-*-custom.xml has a syntax error in it and the service won't start as a result.
xmllint can be used to validate the structure of the index files to makes sure they pass that test before you restart the service.
xmllint validate /etc/netwitness/ng/index-concentrator-custom.xml
Hope that helps someone when creating or modifying their custom index files