Thanks, Andre, for replying.
Following your proposal the overall Genre Pop/Rock would show all albums of the genres Indie Rock, Alternative Rock, Rock'n'Roll etc. in a flat list.
What I want is the overall Genres (e.g. Pop/Rock or Electronic) to be an additional level, which gives access to the particular genre categories defined by tags (e.g. Indie Rock, Alternative Rock, Rock'n'Roll etc.).
Perhaps I should have been more precise beforehand.
Regards