sub tree_iterator {
    return undef unless @stack;
    my $tos = pop @stack;
    return $tos unless ref $tos eq 'ARRAY';
    my ($L, $R) = @$tos;
    print "ENTER ($L, $R)\n";
    while (1) {
        print "LOOP: ($L, $R)\n";
        push @stack, $R if defined $R;
        if (defined $L) {
            return $L if ref $L ne 'ARRAY';
            ($L, $R) = @$L;
            redo;
        }
        else {
            ($L, $R) = @{pop @stack};
        }
    }
}