code: public TreeNode<T> findByContent(T content) {
  		Optional<TreeNode<T>> opt = TreeStream.breadthFirst(treeDef(), this).filter(node -> node.getContent().equals(content)).findFirst();
  		if (opt.isPresent()) {
  			return opt.get();
  		} else {
  			throw new IllegalArgumentException(this.toString() + " has no child with content " + content);
  		}
  	}
docs: Searches breadth-first for the TreeNode with the given content.
  
  @throws IllegalArgumentException if no such node exists