anchor.diff:  
   (deftype signed-array-index ()
     "A (possibly negated) array index."
  -  '#.(let ((limit array-dimension-limit))
  +  '#.(let ((limit (1- array-dimension-limit)))
          `(integer (,(- limit)) (,limit))))
  +
  +(deftype signed-array-length ()
  +  "A (possibly negated) array length."
  +  '#.(let ((limit (1- array-dimension-limit)))
  +       `(integer ,(- limit) ,limit)))
   
   (defsubst sequence? (x)
     (typep x 'sequence))
  
positive.code: the same type.
  
  If N is negative, then |N| elements are dropped from the end of SEQ."
    (check-type n signed-array-index)
    (seq-dispatch seq
      (if (minusp n)
          (butlast seq (abs n))
  
negative.code:       `(remove ,item ,seq :test-not ,test ,@(remove-from-plist args :test))
        decline))
  
  (-> single (sequence) boolean)
  (defsubst single (seq)
    "Is SEQ a sequence of one element?"
    (seq-dispatch seq
  