(defun f (x class &optional
          (index      (table-class (xindex-table x)))
          (range      class)
          (access     #'counted-n)
          (if-missing 0))
  (let ((tmp (gethash `(,class ,index ,range)
                      (xindex-counts x))))
    (if tmp
        (funcall access tmp)
        if-missing)))

(defun uses (x class index range)
  (f x class index range #'counted-all nil))

;(deftest test-xindex-f ()
;  (unless (fboundp 'weather2) (loaddata 'weather2))
;  (let ((x (cross-index (weather2))))
;    (check
;      (= (f x 'no) 5)
;      (= (f x 'no 0 'rainy) 2))))