[r3dey3]

I'm enjoying this; but it looks like a few of the levels might be out of order. In State Selector uses Decoder in the library but it is before decoder. Then in the Arithmetics the "Bus" comes into play before it's described (I forget exactly which ones are out of order)

[sergex]

Doh, you are correct about selector - the decoder should no be available here (and is not needed). And the bus definitely needs an introduction.