* ex::to_rational() provides a way to extend the domain of functions like
gcd() and divide() that only work on polynomials or rational functions (the
good old ex::subs() method reverses this process)
+* Calling diff() on a function that has no derivative defined returns the
+ inert derivative function "Derivative".
* Several new timings in the check target. Some of them may be rather rude
at your machine, feel free to interrupt them.