sub genIterator {
    my( $root ) = @_;
    my @stack = ( $root, 0 );
    return sub {
        while( @stack ) {
            my $index = pop @stack;
            my $tree =  pop @stack;
            if( ref( $tree->[$index] ) ) {
                push( @stack, $tree, $index+1, $tree->[$index], 0 );
            } elsif( $index < @$tree ) {
                push( @stack, $tree, $index+1 );
                return $tree->[$index];
            }
         }
         return;     # empty stack
    }
}