freeSlotsTBMChan :: TBMChan a -> STM Int
freeSlotsTBMChan (TBMChan _closed limit _chan) =
    readTVar limit
freeSlotsTBMChan :: TBMChan a -> STM Int
freeSlotsTBMChan (TBMChan _closed limit _chan) =
    readTVar limit
freeSlotsTBMChan (TBMChan _closed limit _chan) =
    readTVar limit