docs: public void build() {
          Preconditions.checkState(!isBuilt);
  
          if (pendingItemAdj.size() > 0) {
              for (InvoiceItem item : pendingItemAdj) {
                  addExistingItem(item, true);
              }
              pendingItemAdj.clear();
          }
          for (SubscriptionItemTree tree : subscriptionItemTree.values()) {
              tree.build();
          }
          isBuilt = true;
      }