3. A method according to claim 1, wherein at least one of said check words is derived by a primitive polynomial generator using a function of an extension field of a common generator polynomial.
3. A method according to claim 1, wherein at least one of said check words is derived by a primitive polynomial generator using a function of an extension field of a common generator polynomial. 4. A method according to claim 3 wherein one of said check words is derived by modulo-2 addition and the remaining said check words are derived by respective primitive polynomial generators, each using a respective function of an extension field of a common generator polynomial.