(define $common-seqs
  (lambda [$xs $ys]
    (match-all [xs ys] [(list char) (list char)]
      [[(loop $i [1 $n] <join _ <cons $c_i ...>> _)
        (loop $i [1 ,n] <join _ <cons ,c_i ...>> _)]
       (map (lambda [$i] c_i) (between 1 n))])))

(define $lcs (compose common-seqs rac))