Typo: in the user code example, I think the second should be in rather than out. This is pretty cute - looks useful in practice. I think you can slightly enha

Typo: in the user code example, I think the second should be in rather than out. This is pretty cute - looks useful in practice. I think you can slightly enhance the probability of the compiler making the result efficient by using a switch statement, like: goto setup1; got_value: switch (value) { setup1: ...setup 1...; goto setup2; case 1: ...; break; } etc. In C++, you could do something fairly equivalent with lambdas, without having to involve the "evil" preprocessor.