opendir DIR, $dir
    or die "Couldn't read '$dir' : $!";
  my @all_files = map { lc $_ } readdir DIR;
  closedir DIR;
  my %did = map { /(.*)\.did$/ and ($1 => 1) } grep { /\.did$/ } @all_files;
  my @files = grep { /(.*)\.mrg$/ and not $did{$1} } @all_files;