20. The apparatus recited in claim 18 , further comprising: means for calculating the at least one distance metric that quantifies the semantic similarity between the individual rules in the first grammar and the individual rules in the second grammar according to a normalized Levenshtein distance between a first rule in the first grammar and a second rule in the second grammar that have a minimum difference in length.
18. The apparatus recited in claim 13 , wherein the one or more distance metrics include at least one distance metric that quantifies a semantic similarity between individual rules in the first grammar and individual rules in the second grammar.