sub mixedgray_it {
	my @w = @_;
	my @m = map { 0+@$_ } @w;
	die "all items must have at least two positions"
		if grep {$_<2} @m;
	my @a = (0) x @m;
	my @f =  0 .. @m;
	my @o = (1) x @m;
	my $done;
	return sub {
		return undef if $done;
		my $rv = [map {$w[$_][$a[$_]]} 0..$#w];
		if ($f[0]==@w) { $done=1; return $rv }
		my $j = $f[0]; $f[0] = 0;
		$a[$j] += $o[$j];
		if ($a[$j]==0 || $a[$j]==$m[$j]-1) {
			$o[$j] = -$o[$j];
			$f[$j] = $f[$j+1];
			$f[$j+1] = $j+1;
		}
		return $rv;
	}
}

use Data::Dump;
my $it = mixedgray_it(['a','b','c'],[1,2]);
while (defined( my $c = $it->() ))
	{ dd $c }

__END__

["a", 1]
["b", 1]
["c", 1]
["c", 2]
["b", 2]
["a", 2]