Used by the routine at DO_ATTACK.

The random number from RANDOM is added with ADD A,B, and a carry is taken as going past 255: then 0 if the number was negative, 255 if not. But a negative number is a byte of 246 to 255, which carries whenever the true result is fine, so every downward jostle of a value of 10 or more returns 0; below 10 a sum that really goes under 0 does not carry, and comes out as 246 or more. And RANDOM gives a negative number only about one time in 25 for a limit of 10 (see its description), so in practice a blow or a guard goes up by 0 to 10, and about one time in 25 is 0 -- and a guard of 0 loses to any blow over 16. Measured in the simulator: 2000 jostles of 104 came out 104 to 114, or 0 (71 times), never 94 to 103. Watched in the emulator: the vicious warg (strength 55) killed the player (defence 64) with one blow when the player's guard came out 0, which a true jostle could never allow.