I was going to ask about this. Since apparently, all three genders are acceptable, how do we know which one to use? My personal inclination would be to want to use neuter (and in fact that's what we see here), but since masculine and feminine generally take the same form (-en) in Bøkmal Norsk, I could, purely statistically speaking, be wrong up to 66.7% of the time. ;-)
Seriously though: if all three are acceptable, and unless there are different meanings for the different gendered forms (as with ting - which, if it is the case, wasn't made clear in the Tips and Notes), then I suppose theoretically one would almost never be wrong. I'm just trying to come up with an example where only masculine (or feminine) would be acceptable. For example, I could say "Den mannen er søppel" ('that man is trash'), so we'd expect the masculine form, but søppel doesn't take an article in that sentence - definite or indefinite - so it doesn't really matter.
For that matter, is søppel even countable? Can it take an indefinite article, or will we never see that?
Can someone please help me come up with an example in which either the masculine or feminine form of søppelen is not only acceptable, but required? Tusen takk!