jashkenas/underscore — pull request #1474 by braddunbar (2014-02-13 23:01:51+00:00)
Title: Union and Difference Accept Only Arrays

Based on #1428, I tweaked things a bit more (comments inline).

@michaelficarra I also dislike the extra boolean argument but I'm not sure what to do about it as all the other methods I can think of are a performance compromise (options hash, separate filter). I'm not a big fan of co-opting `_.flatten` at all but it otherwise we're essentially including it twice. Suggestions welcome!