5. A method as in claim 2 wherein said model context has M bits of coding and N bits of history decision with value H(A), and the coding context is assigned an index (2**N)A+H(A), if the N bits were the low order bits of the coding context index.
2. A method as in claim 1 wherein the compressing of data includes an arithmetic coding decision using a given model context assigned an index A which is expanded into a context A, N, where N is the number of previous decisions for said given model context used in the conditioning of the arithmetic coding decision.