Creating a new icon

What do you do when you need to convey some information using iconography, and no pre-existing design pattern seems to fit? This was a challenge we recently faced whilst modifying the user interface on a piece of software.

The section of the UI (user interface) was a nested collection of tick-boxes. That is to say, there are parent tick-boxes, and a little [+] symbol on the left of each to click, whereupon the child-items appear, and some of these may also have child-items and so on. What we needed was a way of indicating to the user when a child-item of a tick-box was selected, even though the parent item was not. And given that some of the categories ran up to 5 levels deep, we couldn't simply rely on the user opening out the tree of items to check what was or wasn't ticked.