code: public static int getTwoTermFreq(Term fromTerm, Term toTerm) {
          Nature from = fromTerm.natrue();
          Nature to = toTerm.natrue();
          if (from.index < 0 || to.index < 0) {
              return 0;
          }
          return NATURETABLE[from.index][to.index];
      }
docs: 获得两个term之间的频率
  
  @param fromTerm
  @param toTerm
  @return