code: @Override
  	public boolean isValid() {
  		Boolean valid = _valid;
  		if (valid == null) {
  			valid = _chromosomes.forAll(Verifiable::isValid);
  			_valid = valid;
  		}
  
  		return _valid;
  	}
docs: Test if this genotype is valid. A genotype is valid if all its
  {@link Chromosome}s are valid.
  
  @return true if this genotype is valid, false otherwise.