What is the best approach for setting flag in an method? I could set a global an book keep but then there can only be one insertion method at the time. I could also pass it as an extra parameter in the method call but that would pollute the signature. There is probably a better way to do this.

2.4 PFDS

(define-condition element-already-present (error)
  ((text :initarg :text :reader text)))

(defmethod insert :around (element (node node))
  "Wrap the function call around a catch statement so to prevent unnecesary
copying when element already belongs in the tree. When run for the first time,
setup handler case of the element already present."
  (cond ((first-call
          (handler-case
              (progn
                (let ((result (call-next-method)))
                  result))
            (element-already-present () node)))
         (t (let ((result (call-next-method)))
              result)))))

(defmethod insert (element (node node))
  (cond
    ((ord-lt element (element node))
     (make-instance 'node
                    :left (insert element (left-branch node))
                    :element (element node)
                    :right (right-branch node)))
    ((ord-gt element (element node))
     (make-instance 'node
                    :left (left-branch node)
                    :element (element node)
                    :right (insert element (right-branch node))))
    ((ord-eql element (element node))
     (error 'element-already-present :text "Don't copy in vain."))
    (t node)))