(,fraction? . "fraction, as pair")
(,grob-list? . "list of grobs")
(,index? . "non-negative integer")
+ (,key? . "index or symbol")
+ (,key-list? . "list of indexes or symbols")
+ (,key-list-or-music? . "key list or music")
+ (,key-list-or-symbol? . "key list or symbol")
(,markup? . "markup")
(,markup-command-list? . "markup command list")
(,markup-list? . "markup list")