Quick implementation of sym (+ tests (dogfooding)).

The motivation for implementing this was the test report.

In case of a test not passing, js:alert should not be used IMHO. So I tried to use (println) with a meaningful message (function tested, expected value (the `BoolExpr`) and the actual value) using (sym).

Moreover, I guess that there is a cleaner way to `slice(-1, 1)`.
Jon, since you know the internals better than me, I'd like a hint!