A Simple Tree
[This is a Quicktime Movie, click the Play button to view it. The logical symbols you see in use may be different to the ones you are familiar with (sorry about that, but it is not practical to produce different movies for all the minor variations in symbols). Any differences will not affect the principles being explained here.]
There is one change here from the technique described in the video. With tablets, like the iPad, you select an item by touching it, and un-select it by touching it a second time to toggle it on and off. To select two items, you touch the first one then touch the second.