anchor.diff:         (= (double3-y a) (double3-y b))
          (= (double3-z a) (double3-z b))))
   
  -(cffi:defcstruct double3
  +(cffi:defcstruct (double3 :class double3-c)
     (x :double)
     (y :double)
     (z :double))
  
positive.code:        (= (float4-z a) (float4-z b))
         (= (float4-w a) (float4-w b))))
  
  (cffi:defcstruct float4
    (x :float)
    (y :float)
    (z :float)
  
negative.code:           z (float4-z value)
            w (float4-w value))))
  
  (defmethod cffi:translate-from-foreign (value (type float4-c))
    (cffi:with-foreign-slots ((x y z w) value (:struct float4))
      (make-float4 x y z w)))
  
  