code: @Override
  	public Optional<G> getParent() {
  		checkTreeState();
  
  		return _genes.stream()
  			.filter(g -> g.childStream().anyMatch(this::identical))
  			.findFirst();
  	}
docs: Return the <em>parent</em> node of this tree node.
  
  @return the parent node, or {@code Optional.empty()} if this node is the
  root of the tree
  @throws IllegalStateException if this gene is not part of a chromosome