code: public static Codec<int[], EnumGene<Integer>> ofPermutation(final int length) {
  		require.positive(length);
  
  		return Codec.of(
  			Genotype.of(PermutationChromosome.ofInteger(length)),
  			gt -> gt.getChromosome().stream()
  				.mapToInt(EnumGene::getAllele)
  				.toArray()
  		);
  	}
docs: Create a permutation {@code Codec} of integer in the range
  {@code [0, length)}.
  
  @param length the number of permutation elements
  @return a permutation {@code Codec} of integers
  @throws IllegalArgumentException if the {@code length} is smaller than
  one.