anchor.diff:    If not specified, the <initial-guess> is taken as the zero vector of length <len>."
     (with-foreign-object (k :double len)
       (set-c-vector k initial-guess)
  -    (make-callback c-f len nleqs)
  -    (make-callback-2d c-df len (make-jacobian nleqs))
  +    (make-callback c-f
  +                   len
  +                   nleqs)
  +    (make-callback c-df
  +                   len
  +                   (make-jacobian nleqs)
  +                   set-c-matrix)
       (when (= 1 (newtons-method len k
                                  (get-callback 'c-f)
                                  (get-callback 'c-df)))
  
positive.code:                  do (setf (mem-aref (mem-aref A :pointer i) :double j)
                            (aref rows i j)))))
  
  (defun newton-solver
    (nleqs len
           &optional (initial-guess (make-array len
  
negative.code:   (loop for i below size
          collect (mem-aref vec type i)))
  
  (defmethod set-c-matrix (A rows)
    "Row-wise initialization of c-style matrices."
    (loop for row being the elements of rows
  