(define (make-source s . end)
  (if (string? s)
      (if (null? end)
          (let ((static (reverse (cons #f (reverse (string->list s))))))
            (lambda (x)
              (if (= (length static) 1)
                  (car static)
                  (if (equal? x 'peek)
                      (car static)
                      (if (equal? x 'next)
                          (begin
                            (let ((cur (car static)))
                              (begin
                                (set! static (cdr static))
                                cur)))
                          static)))))
          (let ((static (reverse (cons (car end) (reverse (string->list s))))))
            (lambda (x)
              (if (= (length static) 1)
                  (car static)
                  (if (equal? x 'peek)
                      (car static)
                      (if (equal? x 'next)
                          (begin
                            (let ((cur (car static)))
                              (begin
                                (set! static (cdr static))
                                cur)))
                          static))))))
      (if (vector? s)
          (if (null? end)
              (let ((static (reverse (cons #f (reverse (vector->list s))))))
                (lambda (x)
                  (if (= (length static) 1)
                      (car static)
                      (if (equal? x 'peek)
                          (car static)
                          (if (equal? x 'next)
                              (begin
                                (let ((cur (car static)))
                                  (begin
                                    (set! static (cdr static))
                                    cur)))
                              static)))))
              (let ((static (reverse (cons (car end) (reverse (vector->list s))))))
                (lambda (x)
                  (if (= (length static) 1)
                      (car static)
                      (if (equal? x 'peek)
                          (car static)
                          (if (equal? x 'next)
                              (begin
                                (let ((cur (car static)))
                                  (begin
                                    (set! static (cdr static))
                                    cur)))
                              static))))))
          (if (null? end)
              (let ((static (reverse (cons #f (reverse s)))))
                (lambda (x)
                  (if (= (length static) 1)
                      (car static)
                      (if (equal? x 'peek)
                          (car static)
                          (if (equal? x 'next)
                              (begin
                                (let ((cur (car static)))
                                  (begin
                                    (set! static (cdr static))
                                    cur)))
                              static)))))
              (let ((static (reverse (cons (car end) (reverse s)))))
                (lambda (x)
                  (if (= (length static) 1)
                      (car static)
                      (if (equal? x 'peek)
                          (car static)
                          (if (equal? x 'next)
                              (begin
                                (let ((cur (car static)))
                                  (begin
                                    (set! static (cdr static))
                                    cur)))
                              static)))))))))

(define (peek x)
  (x 'peek))

(define (next x)
  (x 'next))