anchor.diff:    (make-array 0 :element-type 'sv :initial-element (create-sv 0 0)))
   
   (defun compact-sv (sv matrix)
  -  "Take a SV that uses sparse indices of matrix and turn it into one
  +  "Take SV that uses sparse indices of MATRIX and turn it into one
   that uses dense indices."
     (flet ((compact (target source map)
              (loop for i below (length source) do
  
positive.code: (defgeneric map-matrix (function matrix)
    (:documentation "Call FUNCTION for each non-empty cell of MATRIX.
  FUNCTION is of four parameters: ROW, COLUMN, VALUE and DENSE-INDEX
  where DENSE-INDEX is akin to a row major index except it doesn't skips
  over empty cells. DENSE-INDEX is always less than the SIZE-OF the
  MATRIX."))
  
  
negative.code:                             :clip clip :matrix matrix
                              :approximation approximation))
             (add (sv)
               (funcall adder sv approximation)))
        (loop for sv across svd do
              (add sv))
        (loop for n upfrom (length svd) do
  