anchor.diff:                         (reshape u (τ (i) ((1+ i))) interior)
                          (reshape u (τ (i) ((1- i))) interior))))))
         (2
  -       (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:       (rbgs u f h 3) ; solve "exactly"
        (let* ((x (rbgs u f h v1))
               (r (restrict (residual x f h)))
               (c (v-cycle (reshape 0d0 (shape r)) r (* 2 h) v1 v2)))
          (rbgs (α #'+ x (prolongate c)) f h v2))))
  
  
  