"Artists"   artist   : album  : song
"Albums"    album    : song
"Genres"    genre    : artist : album : song
"Composers" composer : album  : song
"Songs"     song
"Search by artist" artist ? artist ~ "" : album : song
"Search by album" album ? album ~ "" : song
"Search by song" song ? song ~ ""
"Search by year" artist ? year = "" : album : song
"Example 1" artist ? year >= "2000" & artist ^ "a" : album : song
"Example 2" genre ? genre ~ "metal" : artist ? year >= "2000" : album ? year >= "2000" : song