(def bidir-table-sided ()
    (with (a-to-b (table)
           b-to-a (table)
           erase-link
           (fn (k-to-v v-to-k k)
             (let v (k-to-v k)
               (= (v-to-k v) nil
                  (k-to-v k) nil)))
           report-error (fn (side) (err:string "bidir-table-sided: not side a or b - " side)))
      (add-attachment
        '= (fn (v side k)
             (let (k-to-v v-to-k)
               (case side
                 a (list a-to-b b-to-a)
                 b (list b-to-a a-to-b)
                   (report-error side))
               (if
                 (no k)
                   (err "bidir-table-sided: can't assign nil key")
                 v
                   (do
                      ;first erase
                      (erase-link k-to-v v-to-k k)
                      (erase-link v-to-k k-to-v v)
                      ;now link them
                      (= (v-to-k v) k)
                      (= (k-to-v k) v))
                 ; else delete
                   (erase-link k-to-v v-to-k k))))
        ; no keys attachment: not a real table, since
        ; we specify a side
        (fn (side k)
          (case side
            a (a-to-b k)
            b (b-to-a k)
              (report-error side))))))

  (= (foo 'a 42) 1)
  (foo 'b 1)