11. A system as in claim 9 further comprising means for assigning the coding context an index 4*A+2(D(A)'+D(A)", where D(A)' and D(A)" are the previous binary coding decisions for said given model context.
9. A system as in claim 10 wherein the compression of data involves an arithmetic coding decision and further comprising means for assigning a given model context an index A; and means for expanding said given model context into a larger coding context where N is the number of previous decisions for said given model context used in the conditioning of the arithmetic coding decision.