第18回オフラインリアルタイムどう書く Haskellで解けなかった

Haskell

http://qiita.com/Nabetani/items/ad47666c2f2f44ada1e7
http://nabetani.sakura.ne.jp/hena/ord18notfork/

module Main where

main = do
	let order = "42873x.3."
	print order

data Customer = Normal | Claimer deriving (Eq, Show)
type Line = [Customer]

checkers :: [Int]
checkers = [2,7,3,5,2]

regist :: [Line] -> [Line]
regist lines = map regist' $ zip [0..] lines where
  regist' nl = dropNormal (checkers !! regidx) line where
    (regidx, line) = nl
    dropNormal 0 l = l
    dropNormal n l@(c:cs) = case c of 
      Normal  -> dropNormal (n-1)  cs
      Claimer -> l

timeElapsed :: String -> [Line]
timeElapsed cs = timeElapsed' cs [] where
  timeElapsed' (c:cs) lines = case c of
    '.' -> timeElapsed' cs $ regist lines
    'x' -> timeElapsed' cs $ lineup 1 Claimer lines
    otherwise -> timeElapsed' cs $ lineup (read [c]) Normal lines

  lineup :: Int -> Customer -> [Line] -> [Line]
  lineup n customer ls = lineup' (replicate n customer) ls where
    lineup' cs ls = undefined

lineup' で詰まった..orz