(defstruct (tree-methods)
  empty-p leaf-p label children
  make-empty make-leaf make-tree)

(defparameter *labelless-nary-tree*
  (make-tree-methods
   :empty-p    (function null)
   :leaf-p     (function atom)
   :label      (constantly nil)
   :children   (lambda (node) node)
   :make-empty (constantly nil)
   :make-leaf  (function identity)
   :make-tree  (lambda (label children)
                 (declare (ignore label))
                 children)))