anchor.diff:  
   (define-condition sheet-supports-only-one-child (error) ())
   
  -(define-condition sheet-already-has-parent (error) ())
  -
   (defmethod sheet-adopt-child :before ((sheet sheet-single-child-mixin)
   				      child)
     (declare (ignorable child))
  
positive.code: (defmethod sheet-disown-child :after (sheet
  				      (child sheet-parent-mixin)
  				      &key (errorp t))
    (declare (ignore sheet errorp))
    (setf (sheet-parent child) nil))
  
  (defmethod sheet-siblings ((sheet sheet-parent-mixin))
    (when (not (sheet-parent sheet))
  
negative.code: 	 (compose-transformations (sheet-transformation sheet)
  				  (sheet-delta-transformation
  				   (sheet-parent sheet) ancestor)))
  	(t +identity-transformation+)))
    
  (define-condition sheet-is-not-ancestor (error) ())
  
  