+ (cond
+ ((listp seq)
+ (if b
+ (let ((diff (- b a)))
+ (cond
+ ((zerop diff) ())
+ ((minusp diff)
+ (error "Start index must be smaller than end index"))
+ (t
+ (let* ((drop-a (copy-list (nthcdr a seq)))
+ (pointer drop-a))
+ (dotimes (_ (1- diff))
+ (setq pointer (cdr pointer))
+ (when (null pointer)
+ (error "Ending index larger than length of list")))
+ (rplacd pointer ())
+ drop-a))))
+ (copy-list (nthcdr a seq))))
+ ((vectorp seq)
+ (let* ((b (or b (length seq)))
+ (size (- b a))
+ (new (make-array size :element-type (array-element-type seq))))
+ (do ((i 0 (1+ i))
+ (j a (1+ j)))
+ ((= j b) new)
+ (aset new i (aref seq j)))))
+ (t (not-seq-error seq))))
+
+(defun copy-seq (sequence)
+ (subseq sequence 0))
+
+
+;;; Reduce (based on SBCL's version)
+
+(defun reduce (function sequence &key key from-end (start 0) end (initial-value nil ivp))
+ (let ((key (or key #'identity))
+ (end (or end (length sequence))))
+ (if (= end start)
+ (if ivp initial-value (funcall function))
+ (macrolet ((reduce-list (function sequence key start end initial-value ivp from-end)
+ `(let ((sequence
+ ,(if from-end
+ `(reverse (nthcdr ,start ,sequence))
+ `(nthcdr ,start ,sequence))))
+ (do ((count (if ,ivp ,start (1+ ,start))
+ (1+ count))
+ (sequence (if ,ivp sequence (cdr sequence))
+ (cdr sequence))
+ (value (if ,ivp ,initial-value (funcall ,key (car sequence)))
+ ,(if from-end
+ `(funcall ,function (funcall ,key (car sequence)) value)
+ `(funcall ,function value (funcall ,key (car sequence))))))
+ ((>= count ,end) value)))))
+ (if from-end
+ (reduce-list function sequence key start end initial-value ivp t)
+ (reduce-list function sequence key start end initial-value ivp nil))))))
+
+(defun elt (sequence index)
+ (when (< index 0)
+ (error "The index ~D is below zero." index))
+ (etypecase sequence
+ (list
+ (let ((i 0))
+ (dolist (elt sequence)
+ (when (eql index i)
+ (return-from elt elt))
+ (incf i))
+ (error "The index ~D is too large for ~A of length ~D." index 'list i)))
+ (array
+ (let ((length (length sequence)))
+ (when (>= index length)
+ (error "The index ~D is too large for ~A of length ~D." index 'vector length))
+ (aref sequence index)))))
+
+(defun mismatch (sequence1 sequence2 &key key (test #'eql testp) (test-not nil test-not-p)
+ (start1 0) (end1 (length sequence1))
+ (start2 0) (end2 (length sequence2)))
+ (let ((index1 start1)
+ (index2 start2))
+ (while (and (<= index1 end1) (<= index2 end2))
+ (when (or (eql index1 end1) (eql index2 end2))
+ (return-from mismatch (if (eql end1 end2) NIL index1)))
+ (unless (satisfies-test-p (elt sequence1 index1) (elt sequence2 index2)
+ :key key :test test :testp testp
+ :test-not test-not :test-not-p test-not-p)
+ (return-from mismatch index1))
+ (incf index1)
+ (incf index2))))
+
+(defun list-search (sequence1 list2 args)
+ (let ((length1 (length sequence1))
+ (position 0))
+ (while list2
+ (let ((mismatch (apply #'mismatch sequence1 list2 args)))
+ (when (or (not mismatch) (>= mismatch length1))
+ (return-from list-search position)))
+ (pop list2)
+ (incf position))))
+
+(defun vector-search (sequence1 vector2 args)
+ (let ((length1 (length sequence1)))
+ (dotimes (position (length vector2))
+ (let ((mismatch (apply #'mismatch sequence1 (subseq vector2 position) args)))
+ (when (or (not mismatch) (>= mismatch length1))
+ (return-from vector-search position))))))