Those are always edge cases. I would be hesitant to provide a simple rule on how to deal with these kinds of answers, although one thing should be clear: **"You cannot do it" *can* be a valid answer.**

However in this particular instance it's clearly this:

> It doesn't solve the real problem. If OP asks "Is <XYZ> possible", he really means "How do I do <XYZ>".

In general, it doesn't make a lot of sense to ask whether something is possible. Asking *how to achieve it* is always preferred, and if the answer is, "it's not possible", then so be it. Clearly, in this case, the OP needs to do something very specific – a workflow in GIMP – and just telling them you can't do it is

- not particularly helpful
- maybe not even true (who knows, maybe it's possible but the answerer hasn't found a way to)

So, this question here can't be compared with others where you can actually *prove* that something is not possible (e.g. "How can I write data to a CD-ROM?" has a definitive "it's not possible" answer).