Yeah, I think the convention should be to return the smallest valid index such that the predicate is satisfied, or an error value if the predicate isn't satisfi

Yeah, I think the convention should be to return the smallest valid index such that the predicate is satisfied, or an error value if the predicate isn't satisfied. As you point out, it's the same as when a non-empty string appears multiple times in a given string. So I think 0 is the only reasonable result in this case, assuming 0-based indexing.