1. In a communication system wherein information is transmitted in the form of a data word representing the information and a check word for error detection, wherein the data word and the check word each include a plurality of code symbol groups, and wherein symbol groups of a received data word are processed at a node of the communications system to produce symbol groups of a modified data word, an apparatus for producing a modified check word to conform with the modified data word, said apparatus comprising: first means for processing a symbol group of the receiving data word and a symbol group of the modified data word after said modified data word symbol group is produced and for producing a comparison symbol group in response thereto; means for encoding said comparison symbol group to produce an encoded symbol group; second means for processing said encoded symbol group and a symbol group of the received check word after said encoded symbol group is produced, to produce a symbol group of the modified check work.
6. The apparatus of claim 1 wherein the second means performs serial symbol subtraction on each symbol of the encoded symbol group and each symbol of the received check word symbol group to produce each symbol of the modified check word symbol group, and wherein the subtraction is performed over the field of numbers used to construct the code symbols.