When trying alternatives, if elaborating the first introduces unresolved type classes, consider this a failure and elaborate the second. Normally, type classes should resolve as soon after application as possible. (aside: we need to write down precise rules for type-directed disambiguation; they are not straightforward due to the possibility of delaying type class resolution, auto implicits and unresolved unification problems. There is a trade off between successful disambiguation and efficient enough compilation, so we should write down what the current trade off is.)