We introduce here a specialized representation for p. Let $ be a fresh symbol that does not occur in p. We define the characteristic string of p as the string where each α j is obtained from α j by removing all the occurrences of symbols in V T .
consider now some occurrence A i of a nonterminal symbol in the right-hand side of p. We define the position set is a variable of the form x i,h for some h. We have σ we are decomposing X into the union of k intervals, with k as small as possible.