sub remove_from_dtgs {
    my ($dtg,$file) = @_;
    for my $i (reverse 0 .. $#files) {
        if ($dtgs{$dtg}[$i] eq $file) {
            splice @{$dtgs{$dtg}}, $i, 1;
            delete $dtgs{$dtg} unless @{$dtgs{$dtg}};
            last;
        }
    }
}