It's true. You can't build a tree on a nest, and opaque pieces would blur that distinction. It's a good thing that TTD is a small game--it's easy to remember the situation of one piece if you try. If you forget, it's not difficult to pick up the piece and look, is it? People on this list seem to be forgetting that when it comes to opaques, but it is a vital concept when playing Volcano with 5 shashes of Treehouse and Volcano caps.