6. A method as set forth in claim 5 wherein for uni-grams (n=1), the estimated probability is given by the recorded frequency divided by the total number of uni-grams; and for n-grams (n.gtoreq.2) the estimated probability is given by a weighted average of the recorded frequency and a probability determined by combining together shorter n-grams.
7. A method as set forth in claim 6 wherein two (n-1)-gram probabilities and an (n-2)-gram probability are combined to form an n-gram probability (p) in accordance with the expression: ##EQU5## where B.sub.1 to B.sub.n are sequentially occurring bytes.