code: public synchronized void add(ConfusionMatrix<T> other) {
          for (T actual : other.matrix.keySet()) {
              Multiset<T> counts = other.matrix.get(actual);
              for (T predicted : counts.elementSet()) {
                  int count = counts.count(predicted);
                  this.add(actual, predicted, count);
              }
          }
      }
docs: Adds the entries from another confusion matrix to this one.