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