anchor.diff:        (pairwise fn (coerce xs 'vector))))
   
   (defun sum (array)
  +  "Return the sum of all of the elements of ARRAY, a generalized array.
  +Operates pairwise for numerical stability."
     (etypecase array
       (bit-vector
        (with-type-dispatch (simple-bit-vector bit-vector) array
  
positive.code: ;;; prefix for generalized array operations.
  
  (defsubst tally (array)
    ;; (reduce #'* (shape array))
    (typecase array
      (sequence (length array))
  
negative.code:     (number array)))
  
  (defun prod (array)
    "Return the product of all of the elements of ARRAY, a generalized array.
  Operates pairwise for numerical stability."
    (etypecase array
      (bit-vector
       (with-type-dispatch (simple-bit-vector bit-vector) array
  