anchor.diff:        (error "Attempting to set the SHEET-TRANSFORMATION of a SHEET-TRANSLATION-TRANSFORMATION-MIXIN to a non translation transformation")))
   
   (defclass sheet-y-inverting-transformation-mixin ()
  -  ((transformation :initform (make-instance 'transformation :myy -1.0)
  +  ((transformation :initform (make-transformation 0 0 0 -1 0 0)
   		   :initarg :transformation
   		   :accessor sheet-transformation)
      ))
  
positive.code: ;;; SHEET geometry classes
  
  (defclass sheet-identity-transformation-mixin ()
    ((transformation :initform (make-instance 'transformation)
  		   :initarg :transformation
  		   :accessor sheet-transformation)
     ))
  
negative.code:       (error "Attempting to set the SHEET-TRANSFORMATION of a SHEET-IDENTITY-TRANSFORMATION-MIXIN to a non identity transformation")))
  
  (defclass sheet-translation-transformation-mixin ()
    ((transformation :initform +identity-transformation+
  		   :initarg :transformation
  		   :accessor sheet-transformation)
     ))
  