I have an indefinite sum, for instance:

> E := Sum(f@g@@k, k=0..n);

If I use D(E), I just get: D(Sum(f@g@@k),k = 0 .. n)

> D(f@g@@k) gives the expected result: D(f)@g@@k * D(g@@k)

Doing this works:

> F := Sum(D(op(E)[1]), op(E)[2]);

   F := Sum(D(f)@g@@k * D(g@@k), k=0..n)

However I want to apply it to complex expressions containing Sums and Products. And I can't check if E is of type indefinite sum. The hasfun(E, Sum...