Whenever I encounter a double negative that gives me trouble, I rephrase it in my own words. PT46.S2.Q10 is a good example of this. You can rephrase "X is not indispensable" to "X is not essential."
@"meaghan.fiske" they did make a comeback around the 70s. PT75, 77, 79, 80, 86, 88... they all have em unfortunately. even though they are rare, I would say do not completely ignore them.