code: public double getFrequency(Pair<T> p) {
          double count = getCount(p);
          return (sum == 0) ? 0 : count / sum;
      }
docs: Returns the frequency of this object pair relative to the counts of all other
  object pair.  This value may also be interpreted as the probability of the
  instance of {@code p} being seen in the items that have been counted.