+sub filename {
+ my ($tracknumber, $tags) = @_;
+ my $title = $tags->{TITLE} // die('title required');
+ my @discnumber = @{$tags->{DISCNUMBER} // []};
+ my @partnumber = @{$tags->{PARTNUMBER} // []};
+ my @version = @{$tags->{VERSION} // []};
+ join('-',
+ (map { two_digits($_) } @discnumber),
+ two_digits($tracknumber),
+ mangle_for_file_name(join('TODO multi-title?', @$title)),
+ # TODO more than one VERSION?
+ (map { mangle_for_file_name($_) } @version),
+ @partnumber,
+ )
+ . '.flac',
+}
+