span element around the list you want to be undisplayable that has the class
tree (in fact it can be element with the correct class).
The clever bit (for me at least) is that it uses a closure to do this. Explaining what a closure is is difficult to do quickly but it involves a function returning a function that is altered by the values passed to the function (the first one).
It’s being used right now to in my sidebar to display “pages”.