1. What do the inputs need to be for the result to be true?
2. Is there a solution where X0 = false?
3. and1 becomes always false,
4. so its other inputs can be eliminated,
5. and and5 also becomes always false,
6. so and8 becomes always false,
7. and so does and9, the result.
8. Therefore we know that X0 must be true,
9. so some more circuittry can be eliminated
10. and we ask a slightly smaller question that the first one:
    Given that X0=true, is there a solution where X1 = false?