anchor.diff:     "Attempted to execute a node which had no function."))
   
   (define-condition ptree-lambda-list-keyword-error (ptree-error)
  -  ((keyword :initarg :keyword :reader ptree-error-keyword))
  +  ((llks :initarg :llks :reader ptree-error-llks))
     (:report
      (lambda (err stream)
        (format stream
  -             "Function arguments in PTREE cannot contain lambda list ~
  -              keywords:~%Found ~a in the definition of ~a"
  -             (ptree-error-keyword err)
  -             (ptree-error-id err))))
  +             "Function arguments in `ptree' cannot contain lambda list ~
  +              keywords.~%In definition of ~a found: ~{~s~^ ~}"
  +             (ptree-error-id err)
  +             (ptree-error-llks err))))
     (:documentation
      "Lambda list keywords found in function definition."))
   
  
positive.code:       (declare (ignore forms))
        (check-type id symbol)
        (check-type args list)
        (when-let (keyword (has-lambda-list-keyword-p args))
          (error 'ptree-lambda-list-keyword-error
                 :id id
                 :keyword keyword))))
    (with-gensyms (tree)
      `(let ((,tree (make-ptree)))
         ,@(loop
  
negative.code:                 root
                  (compute-ptree root ptree (check-kernel)))))))))
  
  (defmacro ptree (defs &body body)
    "Create a ptree using `flet' syntax.
  
  