a method of extracting sections of a data stream , the sections including a set of sequences . each sequence is encoded separately and coupled together to define a section . the method involves determining a combination of at least two sequences of the set , comparing the combination of sequences with sequences in the data stream , and rejecting or accepting extraction of the section of the data stream based upon the result of the comparison . if the combination of sequences does not include a start and end marker for the section , a search for the start and end markers is carried out before the section is extracted .