(let* ((number-of-lanes (the fixnum (number-of-lanes matrix)))
	   (lane-width (the fixnum (lane-width matrix)))
	   (q (the fixnum (floor j lane-width))) ;; Lane index within matrix row
	   (r (- j (the fixnum (* q lane-width)))) ;; Element index within lane
	   (lane-index (+ (the fixnum (* i number-of-lanes)) q)) ;; Lane index within matrix
	   (lane (aref (the simple-array (elements matrix)) lane-index)))

.....

; compiling (DEFMETHOD (SETF MREF) ...)
; file: /tmp/slime8ydM1g
; in: DEFMETHOD (SETF MREF) (SINGLE-FLOAT R-MATRIX-SFP T T)
;     (AREF (THE SIMPLE-ARRAY (LINEAR-ALGEBRA:ELEMENTS LINEAR-ALGEBRA::MATRIX))
;           LINEAR-ALGEBRA::LANE-INDEX)
; ==>
;   (SB-KERNEL:HAIRY-DATA-VECTOR-REF ARRAY SB-INT:INDEX)
;
; note: unable to
;   optimize
; due to type uncertainty:
;   The first argument is a (SIMPLE-ARRAY * (*)), not a SIMPLE-STRING.
;
; note: unable to
;   avoid runtime dispatch on array element type
; because:
;   Upgraded element type of array is not known at compile time.
;
; compilation unit finished
;   printed 2 notes