+* Expressions can be constructed from strings like this:
+ ex e("2*x+y", lst(x, y));
+* 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".