(defun largest-string (s)
  (let* ((str (copy-seq s))
         (len (length str))
         (cur "")
         (i (1- len)))
    (loop while (>= i 0)
          do (progn
               (setf cur (concatenate 'string (string (char str i)) cur))
               (when (= (length cur) 3)
                 (when (string= cur "abb")
                   (setf (char str i) #\b)
                   (setf (char str (+ i 1)) #\a)
                   (setf (char str (+ i 2)) #\a)
                   (cond
                     ((and (< (+ i 4) len) (char= (char str (+ i 4)) #\b))
                      (incf i (+ 4 1)))
                     ((and (< (+ i 3) len) (char= (char str (+ i 3)) #\b))
                      (incf i (+ 3 1)))
                     ((char= (char str (+ i 2)) #\b)
                      (incf i (+ 2 1)))))
                 (if (char= (char str (+ i 1)) #\b)
                     (incf i (+ 1 1))
                     (incf i))
                 (setf cur ""))
               (decf i)))
    str))