sub combo {
    my $symbols = shift;
    my %seen;

    my $combos = NestedLoops([
      $symbols->[0],
      map {
        my $_hold = $_;
        sub {
          [grep {!$seen{join(':', sort @_, $_)}++} @{$symbols->[$_hold]}]
        }
      } 1..$#$symbols
    ]);

    while (my @result = $combos->()) {
        print "@result\n";
    }
    print "\n";
}