docs: @Override
    public final int addDirectoryEntry(E entry) {
      // entry is not a directory entry
      if(entry instanceof LeafEntry) {
        throw new UnsupportedOperationException("Entry is not a directory entry!");
      }
      // this is a not a directory node
      if(isLeaf()) {
        throw new UnsupportedOperationException("Node is not a directory node!");
      }
  
      return addEntry(entry);
    }