anchor.diff:                 ,@options
                  :contents (list ,@(mapcar (lambda (content)
                                              (cond ((and (consp content)
  -                                                       (realp (first content)))
  +                                                       (or (realp (first content))
  +                                                           (member (first content) '(+fill+ :fill))))
                                                     `(list ',(first content)
                                                            ,(second content)))
                                                    (t
  
positive.code: 
   (defmethod initialize-instance :after ((pane xbox-pane) &key contents &allow-other-keys)
     (let ((children (mapcar (lambda (content)
                               (cond ((panep content)
                                      content)
                                     ;;
                                     ((or (eql content +fill+) (eql content '+fill+))
                                      (let ((child (make-instance 'null-pane)))
                                        (setf (constraint-slot pane child 'fill-p) t)
                                        child))
                                     ;;
  
negative.code:  (defmethod xically-content-sr** ((pane box-layout-mixin) child)
     (let (p)
       (cond ((constraint-slot pane child 'fill-p)
              (let ((child-space (compose-space child)))
                (make-space-requirement
                 :major     (space-requirement-major child-space)
                 :min-major (space-requirement-min-major child-space)
                 :max-major +fill+
                 :minor     (space-requirement-minor child-space)
                 :min-minor (space-requirement-min-minor child-space)
                 :max-minor (space-requirement-max-minor child-space))))
             ((setq p (constraint-slot pane child 'fixed-size))
              (let ((sr (compose-space child)))
                (make-space-requirement
  