sub divisors {
  my($n,@factors) = @_;

  my %all_factors;
  foreach my $f (@factors) {
    my @to_add = grep { $_ < $n }
                 map { $f * $_ }
                 keys %all_factors;
    undef @all_factors{ $f, @to_add };
  }
  # 3. Add 1 and n, sort.
  undef $all_factors{1};
  undef $all_factors{$n};
  my @divisors = sort {$a<=>$b} keys %all_factors;
  @divisors;
}