I'll repeat what I said earlier, because apparently it's been ignored judging by the many posts since. It's not that subtracting is harder than adding. It's that the difference is incredibly minor that it shouldn't have any measurable effect on length or difficulty of resolving combat. You're not being asked to factor polynomials here folks. You're being asked to subtract from no more than 20. That's first grade stuff. If you have a difficult time calculating "I rolled a 14, my THAC0 is 18, so I hit AC 4", then I suspect you're having difficulty with every mechanical aspect of the game. I mean, you're constantly doing harder math every time you gain or spend coins (gp, sp, etc).