Used by the routines at JOSTLE, DO_STRIKE and RANDOM_POSITIVE.

Mixes the last result, kept at RANDOM_LAST and seeded from R by START, with the byte the pointer at RANDOM_POINTER has got to -- which steps on 256 bytes every call: INC (IX+1) is its high byte, and the low byte moves on only when the high one wraps, so it sweeps the whole of memory a page at a time -- and one DE bytes past it, and draws again if that repeats the last result. The byte is then halved until it is no more than twice A, and A taken off.

The halving keeps any byte over twice A in the upper half, A to twice A, so the result is nearly always 0 to A: it is negative only when the byte is under A, about one call in 25 for A = 10 (measured through JOSTLE: 71 in 2000).

Measured over 3000 calls each, through RANDOM_POSITIVE: every value from 0 to A comes up, but not evenly -- for A = 4 the ends come up half as often as the middle, and for A = 9 the top three do.