sub genIterator {
    my $tree = shift;
    my @stack = ( [ 0, $tree ], 0 );
    return sub {
        my $work     = shift( @stack );
        while ( ref( $work ) ) {
            my $index    = $work->[0];
            my $worktree = $work->[1];

            if ( $index >= scalar( @{$worktree} ) ) {   # Update 3
                $work = shift( @stack );
                next;
            }
            elsif ( ! ref( $worktree->[$index] ) ) {
                unshift( @stack, [ $index+1, $worktree ] );
                return $worktree->[$index];
            }
            else {
                unshift( @stack, [ $index+1, $worktree ] );
                $work = [ 0, $worktree->[$index] ];
                next;
            }
         }
         return;     # empty stack
    }
}