Bug or badly suboptimal result for derivative

[quinyu]

Okay, now it works; no clue why it did not previously. Either way, even though I can now make the calculator to follow what I would do in that given situation (which is good for checking the procedure), I am still unable to get the calculator do it on its own. Are there any (hidden) switches to diff so as to influence the preference to the possible strategical steps in the evaluation tree?