code: public void union(GeneralSubtrees other) {
          if (other != null) {
              for (int i = 0, n = other.size(); i < n; i++) {
                  add(other.get(i));
              }
              // Minimize this
              minimize();
          }
      }
docs: construct union of this GeneralSubtrees with other.
  
  @param other GeneralSubtrees to be united with this