When I was a kid I played a game on the Apple ][ that years later I realized was secretly an introduction to digital design with propagation delay. In the game, little sparks/pulses would travel down wires, and there were various objects (primitive elements) that would manipulate the pulses (and gates, or gates, delays, relays, etc.) If memory serves, each level had a set of pulses that needed to be generated to progress to the next level. I have no idea what it was called and I've looked for it occasionally without success. Anyone know what I'm talking about?