5. The computer program product of claim 1 , wherein the operations further comprise: generating a code distribution of frequencies of groups of words of the symbols that are merged, each having a number of words, wherein a frequency of each of the groups comprises a sum of the frequencies of the words included in a group according to a word frequency distribution, wherein the symbols that are merged are constructed from the code distribution, and wherein each symbol in the code alphabet maps to the words in one of the groups and has the frequency of the group of words to which the symbol maps.
6. The computer program product of claim 5 , wherein the code alphabet comprises a first code alphabet and wherein the code distribution of frequencies comprises a first code distribution of frequencies, wherein the operations further comprise: generating a second code distribution of frequencies of a merged second number of groups of the symbols in the first code alphabet, wherein a frequency of each of the merged second number of groups comprises a sum of frequencies of the words of the symbols in the first code alphabet merged into a group, wherein a second code alphabet is constructed from the second code distribution, and wherein each symbol in the second code alphabet maps to the words in one of the merged second number of groups and has a frequency of the group of words to which the symbol maps.