Beyond getting a complex number as output: which complex number? (For example, is `i` or `-i` the correct answer to `sqrt(-1)`? If that seems too easy, how about the correct answer to `sqrt(-i)`?) It would be interesting to see a calculator that handled multi-valued functions properly, but I think that would require some major UI (and possibly even theoretical) advances in order to degenerate gracefully to the single-valued case.
