code: public GV3CompressedFunction<T> build() throws IOException {
  			if (built) throw new IllegalStateException("This builder has been already used");
  			if (codec == null) codec = new Codec.Huffman();
  			built = true;
  			if (transform == null) {
  				if (chunkedHashStore != null) transform = chunkedHashStore.transform();
  				else throw new IllegalArgumentException("You must specify a TransformationStrategy, either explicitly or via a given ChunkedHashStore");
  			}
  			return new GV3CompressedFunction<>(keys, transform, values, indirect, tempDir, chunkedHashStore, codec);
  		}
docs: Builds a new function.
  
  @return a {@link GOV3Function} instance with the specified
  parameters.
  @throws IllegalStateException
  if called more than once.