  The set of all uniquely decipherable (UD) codes is partially ordered by
refinement, meaning that all strings in the cruder code can be represented as
concatenations of strings taken from the finer code. The Kraft sum is a
monotone (increasing) function on this poset. In the refinement order, chains
of UD codes having the same Kraft sum are necessarily of the simple sequence
type. A characterization of UD codes in terms of equalities involving Kraft
sums is also given.
