B/X: Ability score adjustments

This is kind of an interesting idiosyncrasy of B/X. Its also one of those odd rules that seemed to make it's way informally into a lot of the 1E campaigns I played bitd, though I remember everyone playing a bit fast and loose with which abilities points could be taken from, as opposed to following the strict guidelines given in B/X.

Its also a mitigating factor to the "3d6-in-order" style of character generation, which some folks seem to lament as not giving characters "high enough" ability scores. Given that only a 13 in a class's Prime Requisite is needed for an xp boost and maybe even a bonus to fighting, saves, or AC, not too many points, on average, even need to be shifted around. And it can make it easier to get those rarer 16's or 18's.