Re: what should I assume to evaluate?

Something is very strange here in maple 9.

restart;
assume(R>0);assume(a>0);assume(a>R);
simplify(int((a-x)*(1/(a-x)-1/sqrt(a^2-2*a*x+R^2)),x=-R..R));

and I get a lot of signum(R~ + a~)

Why is this?
.