computing distances #2

  (let loop-i ((ipts pts)
               (i 0)
               (d 0))
    (if (null? ipts)
      d
      (let loop-j ((jpts pts)
                   (j 0)
                   (d d))
        (if (null? jpts)
          (loop-i (cdr ipts) (+ i 1) d)
          (loop-j (cdr jpts)
                  (+ j 1)
                  (if (= i j)
                    d
                    (+ d (expt (+ (expt (- (caar   ipts) (caar   jpts)) 2)
                                  (expt (- (cadar  ipts) (cadar  jpts)) 2)
                                  (expt (- (caddar ipts) (caddar jpts)) 2))
                               -0.5))))))))