anchor.diff:  	 (compose-transformations (sheet-transformation sheet)
   				  (sheet-delta-transformation
   				   (sheet-parent sheet) ancestor)))
  -	(t (sheet-transformation sheet))))
  +	(t +identity-transformation+)))
     
   (define-condition sheet-is-not-ancestor (error) ())
   
  
positive.code: ;;;; sheet protocol class
  
  (defclass sheet ()
    ((region :initarg :region :accessor sheet-region)
     (enabled-p :initform nil :accessor sheet-enabled-p)))
  
  (defun sheetp (x)
  
negative.code: (define-condition sheet-is-not-child (error) ())
  
  (defmethod sheet-disown-child ((sheet sheet) (child sheet) &key (errorp t))
    (when errorp
      (error 'sheet-is-not-child)))
  
  (defmethod sheet-siblings ((sheet sheet))
    (remove sheet (sheet-children (sheet-parent sheet))))
  