code: def add_chunk(self, chunk_obj):
          
          if chunk_obj.get_id() in self.idx:
              raise ValueError("Chunk with id {} already exists!"
                               .format(chunk_obj.get_id()))
          self.node.append(chunk_obj.get_node())
          self.idx[chunk_obj.get_id()] = chunk_obj
docs: Adds a chunk object to the layer
          @type chunk_obj: L{Cchunk}
          @param chunk_obj: the chunk object