It needs to be done. The only one I know of it & (and maybe |, I never tested it, though), but it has been extremely useful in comparing the contents of lists.
Those are actually fully documented, and the ones that aren't are actually the known, often used operators: -= and +=. Their entries don't cover lists, which are an exception: the result of ListVar = ListVar + X is different from ListVar += X.
In response to Kaioken
Oh, my Dream Maker reference has been messed up for a really long time, and that's the reference I checked before looking. Anyone know how to fix an outdated Dream Maker reference? I'm pretty sure it's IE-related (the reference BYOND stores in the program files folder is correct), but clearing my cookies, cache, etc. didn't do anything.
In response to Jeff8500
Did you try deleting byond from your computer, then re-downloading it? (Just a suggestion)
In response to Megelic
I didn't try uninstalling it, but I've tried reinstalling it, many times.