update cs (SwapLetter x y) =
  let (Just ix) = S.elemIndexL x cs
      (Just iy) = S.elemIndexL y cs
  in S.update ix y (S.update iy x cs)
update cs (SwapLetter x y) =
  let (Just ix) = S.elemIndexL x cs
      (Just iy) = S.elemIndexL y cs
  in S.update ix y (S.update iy x cs)
update cs (SwapLetter x y) =
  let (Just ix) = S.elemIndexL x cs
      (Just iy) = S.elemIndexL y cs
  in S.update ix y (S.update iy x cs)