anchor.diff:    (let ((interior (interior u)))
       (ecase (rank u)
         (1
  -       (fuse* (reshape 0 (shape u))
  +       (fuse* (reshape 0d0 (shape u))
                 (α #'- (reshape b interior)
                    (α #'* (/ 1 (* h h))
                       (α #'-
  
positive.code:                        (reshape u (τ (i j) (i (1+ j))) interior)
                         (reshape u (τ (i j) (i (1- j))) interior))))))
        (3
         (fuse* (reshape 0 (shape u))
                (α #'- (reshape b interior)
                   (α #'* (/ 1 (* h h))
                      (α #'-
  
negative.code:                        (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 #'-
  