nit/style question: I would normally do: "/* previous */ false" for readability and since it doesn't seem like a jsdoc/doxygen/etc. block.

It also does beg the question of whether we should just use the string 'previous' instead.  Of course, the meaning of all that is relative anyways.  I had used -1 and +1 originally in the bug discussion since we get consistency with the slice's ordering (and the DOM node ordering).  Doesn't matter too much to me.  Just pontificating :)