Insert a tag with a unique identifier into a XML

I'm able to add a new node with a custom fixed value to my XML file in this way, with xmlstarlet : xmlstarlet ed --subnode "/legge190/data/lotto" --type elem -n newsubnode \ -v "myvalue" But how to add a unique identifier? If I try to add a unique identifier using generate-id(.) xmlstarlet ed --subnode "/legge190/data/lotto" --type elem -n newsubnode \ -v "generate-id(.)" I do not have an id value, but the generate-id(.) string. Thank you