force :: IS -> ()
force (I.IxSet ll) = seq (go ll) ()
  where
    go [] = ()
    go (I.Ix m _:xs) = m `seq` go xs
force :: IS -> ()
force (I.IxSet ll) = seq (go ll) ()
  where
    go [] = ()
    go (I.Ix m _:xs) = m `seq` go xs
force (I.IxSet ll) = seq (go ll) ()
  where
    go [] = ()
    go (I.Ix m _:xs) = m `seq` go xs