(defun chars-in-common (list-of-strings)
  (let ((chars (make-hash-table :test #'equal))
        (res ()))
    (loop :for string :in list-of-strings
          :do (loop :for char :across string
                    :do (if (numberp (gethash char chars))
                            (incf (gethash char chars))
                            (setf (gethash char chars) 1))))
    (maphash (lambda (key val)               
               (when (>= val (length list-of-strings))
                 (push key res)))
             chars)
    res))