sub find_holes {
  my @list = @{ shift() };
  @list = sort { $a <=> $b } @list;
  my $low = $list[0];
  my $high = $list[-1];
  my @notfound;
  @notfound[$low..$high] = ($low..$high);
  $notfound[$_] = 0 for @list;
  my @vacancies = grep {$_} @notfound;
  return \@vacancies;
}