(defun delta-E-CIE-2000 (L1 a1 b1 L2 a2 b2)
  "0 - 1A normally invisible difference 1 - 2Very small difference,
   only obvious to a trained eye 2 - 3.5Medium difference, also
   obvious to an untrained eye 3.5 - 5An obvious difference> 6A very
   obvious difference."
  (let* ((L′-mean (/ (+ L1 L2) 2))
	 (C1 (sqrt (+ (expt a1 2) (expt b1 2))))
	 (C2 (sqrt (+ (expt a2 2) (expt b2 2))))
	 (C-mean (/ (+ C1 C2) 2))
	 (G (* 1/2 (- 1 (sqrt (/ (expt C-mean -7) (+(expt C-mean -7) (expt 25 7)))))))
	 (a′1 (* a1 (1+ G)))
	 (a′2 (* a2 (1+ G)))
	 (C′1 (sqrt (+ (expt a′1 2)(expt b1 2))))
	 (C′2 (sqrt (+ (expt a′2 2)(expt b2 2))))
	 (C′-mean (/ (+ C′1 C′2) 2))
	 (h′1 (rad->deg (atan b1 a1)))
	 (h′2 (rad->deg (atan b2 a2)))
	 (h′-delta (abs (- h′1 h′2)))
	 (H′-mean (/ (+ h′1 h′2) 2);(if (> h′-delta 180) (/ (+ h′1 h′2 360) 2) )
		  )
	 (Tee (+ 1 (* -0.17 (cos (deg->rad (- H′-mean 30))))
	         (* 0.24 (cos (deg->rad (* H′-mean 2))))
	         (* 0.32 (cos (deg->rad (+ (* H′-mean 3) 6))))
	         (* -0.20 (cos (deg->rad (- (* H′-mean 4) 63))))))
	 (Δh′ (let ((dh (abs (- h′2 h′1))))
		     (cond ((<= dh 180)(- h′2 h′1))
			   ((and (> dh 180)(<= h′2 h′1))(+ (- h′2 h′1) 360))
			   (t (- h′2 h′1 360)))))
	 (ΔL′ (- L2 L1))
	 (ΔC′ (- C′2 C′1))
	 (ΔH′ (* 2 (sqrt (* C′1 C′2)) (sin (deg->rad (/ Δh′ 2)))))
	 (Sl (1+ (/ (* 0.015 (expt (- L′-mean 50) 2))
		    (sqrt (+ 20 (expt (- L′-mean 50) 2))))))
	 (Sc (1+ (* 0.045 C′-mean)))
	 (Sh (1+ (* 0.015 C′-mean Tee)))
	 (Δtheta (* 30 (exp (- (expt (/ (- H′-mean 275) 25)2)))))
	 (Rc (* 2 (sqrt (/ (expt C′-mean -7) (+(expt C′-mean -7) (expt 25 7))))))
	 (Rt (- (* Rc (sin (* 2 Δtheta)))))
	 (Kl 1)
	 (Kc 1)
	 (Kh 1))
    (utils:fp 'C′1 C′1
	      'C′2 C′2
	      'h′1 h′1
	      'h′2 h′2
	      'h′-delta h′-delta
	      'H′-mean H′-mean
	      'G G
	      'Tee Tee
	      'Sl Sl
	      'Sc Sc
	      'Sh Sh
	      'Rt Rt)
    (sqrt (+ (expt (/ ΔL′ (* Kl Sl)) 2)
	     (expt (/ ΔC′ (* Kc Sc)) 2)
	     (expt (/ ΔH′ (* Kh Sh)) 2)
	     (* Rt (/ ΔC′ (* Kc Sc))(/ ΔH′ (* Kh Sh)))))))