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