why is the antiderivative of f1(x) equal to 0 at x=0, but the antiderivative of f2(x) is undefined at x=0? are f1(x) and f2(x) not identical functions? f1(x):=1/(1-x); integrate(f1(x),x); f2(x):=-1/(x-1); integrate(f2(x),x);