Finds the depth limit for items in the parent select.
$book_link: A fully loaded menu link that is part of the book hierarchy.
The depth limit for items in the parent select.
function _book_parent_depth_limit($book_link) {
return MENU_MAX_DEPTH - 1 - ($book_link['mlid'] && $book_link['has_children'] ? menu_link_children_relative_depth($book_link) : 0);
}