iterator: advance the tree iterator smartly While advancing the tree iterator, if we advance over things that we aren't interested in, then call `current`. Which may *itself* call advance. While advancing the tree iterator, if we advance over things that we aren't interested in, then call `current`. Which may *itself* call advance. While advancing the tree iterator, if we advance over things that we aren't interested in, then call `current`. Which may *itself* call advance. While advancing the tree iterator, if we advance over things that we aren't interested in, then call `current`. Which may *itself* call advance. While advancing the tree iterator, if we advance over things that we aren't interested in, then call `current`. Which may *itself* call advance. Error: stack overflow.