21. Apparatus as in claim 20 wherein said assigning means includes means for representing the value p'(y.sub.k) for a given y.sub.k in a given phone machine by the maximum p(y.sub.k) value for the given y.sub.k in the given phone machine.
20. Apparatus as in claim 18 wherein the length of the string of labels generable by each phone machine is characterized (a) as being between a minimum length and a maximum length and (b) as having a uniform length distribution wherein the probability for each length in the length distribution between the minimum and maximum length is a uniform value; the probability of label lengths less than the minimum length or greater than the maximum length being set as zero.