import Control.Concurrent.Chan
import Data.List
import Control.Concurrent.MVar
import Control.Concurrent
import Control.Monad


data Clock = Clock (MVar Integer)

getTicket :: Clock -> IO (Integer)
getTicket (Clock m) = do
          old <- takeMVar m
          putMVar m (old + 1)
          return old

--         c1
--      -------->
--     p-------->q
--      ^  c2   /
--    c4 \     / c3
--        \   v
--          r
main :: IO ()
main = do
    m <- newEmptyMVar
    clock <- Clock (m)
    putStrLn "Hello"