code: public void addAll(HyperLogLog other) {
          if (this.sizeof() != other.sizeof()) {
              throw new RuntimeException("Cannot merge estimators of different sizes");
          }
          registerSet.merge(other.registerSet);
      }
docs: Add all the elements of the other set to this set.
  <p/>
  This operation does not imply a loss of precision.
  
  @param other A compatible Hyperloglog instance (same log2m)
  @throws CardinalityMergeException if other is not compatible