docs: public void appendChild(final Node child) {
          if (child instanceof Level) {
              // Append the level
              levels.add((Level) child);
              nodes.add(child);
              if (child.getParent() != null) {
                  child.removeParent();
              }
              child.setParent(this);
          } else if (child instanceof SpecTopic) {
              appendSpecTopic((SpecTopic) child);
          } else if (child instanceof CommonContent) {
              // Append the common content
              commonContents.add((CommonContent) child);
              nodes.add(child);
              if (child.getParent() != null) {
                  child.removeParent();
              }
              child.setParent(this);
          } else {
              nodes.add(child);
              if (child.getParent() != null) {
                  child.removeParent();
              }
              child.setParent(this);
          }
      }