anchor.diff:            :documentation "The start of the iteration, either an index value or integer value to mark the start of iteration")
      (index :initform 0
             :documentation "the current index of the iteration") 
  -   (end :initform -1 :initarg :end
  -        :documentation "The end of the iteration, -1 denotes an infinite iteration, for lists/sequences, the sequence length limits the iteration")
  +   (length :initform -1 :initarg :length
  +        :documentation "The length of the iteration, -1 denotes an infinite iteration, for lists/sequences, the sequence length limits the iteration")
      (increment :initform 1 :initarg :inc
                 :documentation "the increment of the iteration")
      (id :initform #'identity :initarg :id
  
positive.code:      while (and i j)
       collect (cons i j)))
  
  (defmacro make-iterator(&key (start 0) (end -1) (inc 1)
                            (id #'identity) (values ()) (cyclic nil) (reverse nil))
    `(let ((tmp (make-instance 'iterator :start ,start
                               :end (if (null ,values)
                                        ,end
                                        (1- (length ,values)))
                               :inc ,inc :id ,id :values ,values :cyclic ,cyclic)))
       (with-slots (start end index values increment comparer bound-check) tmp
         (when (not (listp values))
           (setf comparer #'(lambda(n seq) (elt seq n))))
         (if ,reverse
           (setf start end
                 index end
                 end ,start
                 bound-check #'>=
                 increment (* -1 ,inc))
           (setf index start)))
       tmp))
           
  (defmacro with-iterator((name &key (start 0) (end -1) (increment 1)
                                (values nil) (id #'identity)) &body body)
    `(let ((,name (make-iterator :start ,start
                                 :end ,end :inc ,increment
                                 :values ,values :id ,id)))
       ,@body))
  
  (defmethod to-string((iter iterator))
    (with-slots (start end increment id quiet values cyclic) iter
      (format T "start: ~A, end: ~A, quiet: ~A, inc: ~A, cyclic: ~A, values: ~A ~%"
              start end quiet increment cyclic values)))
  
negative.code:    (quiet :initform T :initarg :quiet
            :documentation "Whether to raise an error when iterations are exhausted")
     (cyclic :initform nil :initarg :cyclic
             :documentation "Whether the iteration is cyclic, in case where initial-contents are supplied the sequence re-iterated otherwise the iteration resets the index to start")
     (initial-contents :initform nil :initarg :initial-contents
                       :documentation "A list/sequence of initial-contents to be iterated")
     (initial-element :initform nil :initarg :initial-element
                      :documentation "When supplied and no initial-contents supplied, iteration returns the value instead of the index")
     (bound-check :initform #'<= :initarg :from-end
                  :documentation "When T, direction of iteration is from-endd")
     (comparer :initform #'nth
               :documentation "Function to access to individual members of a initial-contents, #'nth for lists, #'elt for others")))
  
  (defmethod reset ((iter iterator))
    (with-slots (start index) iter
      (setf index start)))
  
  (defmethod next ((iter iterator))
    (with-slots (index length increment id quiet cyclic initial-element
                       comparer bound-check initial-contents) iter
      (if (or (funcall bound-check index length) (eq length -1))
          (let ((current
                 (funcall id (cond
                               ((not (null initial-contents)) (funcall comparer index initial-contents))
                               ((not (null initial-element)) initial-element)
                               (T index)))))
            (incf index increment)
            current)
          (if cyclic
  