aggregated-distance

          (let loop-i ((ixs xs)
                       (iys ys)
                       (izs zs)
                       (i 0)
                       (d 0))
           (cond
             ((null? ixs)
              d)
             (else
              (let loop-j ((jxs xs)
                           (jys ys)
                           (jzs zs)
                           (j 0)
                           (dj d))
                  (cond
                    ((null? jxs)
                     (loop-i (cdr ixs) (cdr iys) (cdr izs) (+ i 1) dj)))))
                    ((= i j)
                     (loop-j (cdr jxs) (cdr jys) (cdr jzs) (+ j 1)
                             dj))
                    (else
                     (loop-j (cdr jxs) (cdr jys) (cdr jzs) (+ j 1)
                             (+ dj (expt (+ (expt (- (car ixs) (car jxs)) 2)
                                           (expt (- (car iys) (car jys)) 2)
                                           (expt (- (car izs) (car jzs)) 2))
                                        -0.5)))))