You have user groups which allows for duplicate usernames. The hierarchy also imposes a top level identity authority. In your example,'tld' is responsible for h

You have user groups which allows for duplicate usernames. The hierarchy also imposes a top level identity authority. In your example,'tld' is responsible for hostname,not for username. In my example 'de' is responsible for verifying the user with 'hn' merely verifying against 'de'. Your example is user@fqdn. My idea is <fqui> ,which authoritatively qualifies a user not a user and a domain