code: public CHDMinimalPerfectHashFunction<T> build() throws IOException {
  			if (built) throw new IllegalStateException("This builder has been already used");
  			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 CHDMinimalPerfectHashFunction<>(keys, transform, lambda, loadFactor, signatureWidth, tempDir, chunkedHashStore);
  		}
docs: Builds a minimal perfect hash function.
  
  @return a {@link CHDMinimalPerfectHashFunction} instance with the specified parameters.
  @throws IllegalStateException if called more than once.