The draw_menu function expects three values: an array containing our menu, an array containing the titles we want open when the page loads and an ID for keeping track of things. It's important to note that the ID is passed by reference, meaning that we're working with the original variable rather than making a copy of it. If we didn't pass the ID by reference, the function would get a little confused during recursion and repeat ID's when we don't want it to.
Here we are saying that our root div should be visible (so the rest of the menu is visible), and each additional div should start off hidden. We need to insert the style here manually rather than including it with the rest of the stylesheet.