anchor.diff:          (fuse*
            (reshape u selection)
            (α #'+
  -            (α #'* 0.50d0   (reshape u interior))
  -            (α #'* 0.25d0  (reshape u (τ (i) ((1+ i))) interior))
  -            (α #'* 0.25d0  (reshape u (τ (i) ((1- i))) interior))))
  +            (α #'* 1/2 (reshape u interior))
  +            (α #'* 1/4 (reshape u (τ (i) ((1+ i))) interior))
  +            (α #'* 1/4 (reshape u (τ (i) ((1- i))) interior))))
           (τ (i) ((+ start-1 (/ (- i start-1) 2)))))))
       ((~ start-1 1 end-1 ~ start-2 1 end-2)
        (let* ((selection (~ start-1 2 end-1 ~ start-2 2 end-2))
  
positive.code:   (let ((interior (interior u)))
      (ecase (rank u)
        (1
         (fuse* (reshape 0d0 (shape u))
                (α #'- (reshape b interior)
                   (α #'* (/ 1 (* h h))
                      (α #'-
                         (reshape (α #'* 2.0d0 u) interior)
                         (reshape u (τ (i) ((1+ i))) interior)
                         (reshape u (τ (i) ((1- i))) interior))))))
        (2
         (fuse* (reshape 0d0 (shape u))
                (α #'- (reshape b interior)
                   (α #'* (/ 1 (* h h))
                      (α #'-
                         (reshape (α #'* 4.0d0 u) interior)
                         (reshape u (τ (i j) ((1+ i) j)) interior)
                         (reshape u (τ (i j) ((1- i) j)) interior)
                         (reshape u (τ (i j) (i (1+ j))) interior)
                         (reshape u (τ (i j) (i (1- j))) interior))))))
        (3
         (fuse* (reshape 0d0 (shape u))
                (α #'- (reshape b interior)
                   (α #'* (/ 1 (* h h))
                      (α #'-
                         (reshape (α #'* 6.0d0 u) interior)
                         (reshape u (τ (i j k) ((1+ i) j k)) interior)
                         (reshape u (τ (i j k) ((1- i) j k)) interior)
                         (reshape u (τ (i j k) (i (1+ j) k)) interior)
  
negative.code:         (fuse*
           (reshape u selection)
           (α #'+
              (α #'* 1/8
                 (reshape u interior))
              (α #'* 1/16
                 (reshape u (τ (i j k) ((1+ i) j k)) interior)
                 (reshape u (τ (i j k) (i (1+ j) k)) interior)
                 (reshape u (τ (i j k) (i j (1+ k))) interior)
                 (reshape u (τ (i j k) ((1- i) j k)) interior)
                 (reshape u (τ (i j k) (i (1- j) k)) interior)
                 (reshape u (τ (i j k) (i j (1- k))) interior))
              (α #'* 1/32
                 (reshape u (τ (i j k) (i (1+ j) (1+ k))) interior)
                 (reshape u (τ (i j k) ((1+ i) j (1+ k))) interior)
                 (reshape u (τ (i j k) ((1+ i) (1+ j) k)) interior)
                 (reshape u (τ (i j k) (i (1- j) (1+ k))) interior)
                 (reshape u (τ (i j k) ((1- i) j (1+ k))) interior)
                 (reshape u (τ (i j k) ((1- i) (1+ j) k)) interior)
                 (reshape u (τ (i j k) (i (1+ j) (1- k))) interior)
                 (reshape u (τ (i j k) ((1+ i) j (1- k))) interior)
                 (reshape u (τ (i j k) ((1+ i) (1- j) k)) interior)
                 (reshape u (τ (i j k) (i (1- j) (1- k))) interior)
                 (reshape u (τ (i j k) ((1- i) j (1- k))) interior)
                 (reshape u (τ (i j k) ((1- i) (1- j) k)) interior))
              (α #'* 1/64
                 (reshape u (τ (i j k) ((1+ i) (1+ j) (1+ k))) interior)
                 (reshape u (τ (i j k) ((1+ i) (1+ j) (1- k))) interior)
                 (reshape u (τ (i j k) ((1+ i) (1- j) (1+ k))) interior)
                 (reshape u (τ (i j k) ((1+ i) (1- j) (1- k))) interior)
                 (reshape u (τ (i j k) ((1- i) (1+ j) (1+ k))) interior)
                 (reshape u (τ (i j k) ((1- i) (1+ j) (1- k))) interior)
                 (reshape u (τ (i j k) ((1- i) (1- j) (1+ k))) interior)
                 (reshape u (τ (i j k) ((1- i) (1- j) (1- k))) interior))))
          (τ (i j k) ((+ start-1 (/ (- i start-1) 2))
                      (+ start-2 (/ (- j start-2) 2))
                      (+ start-3 (/ (- k start-3) 2)))))))))
  