docs: public void removeChild(final Node child) {
          if (child instanceof Level) {
              levels.remove(child);
              nodes.remove(child);
              child.setParent(null);
          } else if (child instanceof SpecTopic) {
              removeSpecTopic((SpecTopic) child);
          } else if (child instanceof CommonContent) {
              commonContents.remove(child);
              nodes.remove(child);
              child.setParent(null);
          } else {
              nodes.remove(child);
              child.setParent(null);
          }
      }