[entropy_]

He built an adder, which does always complete(by design). But having the ability to build AND and NOT gates(which he demonstrates) should be enough to build any computer from just those 2 components[1]. I'm not familiar with the game to comment on the correctness of the AND + NOT implementation itself though:

If NOT is modeled by flooding a power plant and causing it to turn off, would it go back to being on after a while if we stop actively flooding it? Ie, If the input goes to 0, does the output go back to 1 or not? If this doesn't hold then it's not a proper NOT gate to begin with(turns 1 to 0 but doesn't turn 0 to 1) and the implementation is flawed.

[1] https://stackoverflow.com/questions/4908893/what-logic-gates...

[tom_mellior]

The sewage plant will stop pumping out sewage if its power is gone. The sewage that floods the power plant will then drain away; it is only flooded if there is a continuous influx of sewage. So yes, it looks like this is a proper NOT gate because the power plant is flooded iff the sewage plant's power is on (modulo propagation delays).