sub divisors {
  my($n,@factors) = @_;
  my %divisors;
  foreach my $l ( powerset(@factors) ) {
    my $d = 1;
    $d *= $_ for @$l;
    undef $divisors{$d};
  }
  my @d = sort { $a<=>$b } keys %divisors;
  @d;
}
sub powerset {@_ ? map { $_,[$_[0], @$_] } powerset(@_[1..$#_]) : [];}