(1) f:=operator 'f
(1)  f
Type: BasicOperator
(2) -> D(sum(f(i),i=a..x),x)
(2)  f(x)                                                   Type: Expression Integer

is nonsense. The problem is in COMBF/dvdsum:

    dvdsum(l, x) ==
      x = retract(y := third l)@SE => 0
      k := retract(d := second l)@K
      differentiate(h := third rest rest l,x) * eval(f := first l, k, h)
        - differentiate(g := third rest l, x) * eval(f, k, g)
             + opdsum [differentiate(f, x), d, y, g, h]

Although this -- d/dx sum_i^x f(i) = f(x) -- is a nice try, it is wrong. It seems that the order of magnitude is roughly correct, but that's about it.

Unfortunately, I do not know how to return the thing unevaluated, so I cannot fix the bug yet