fun center s width =
  let
    open StringCvt

    val len = size s
    val m = abs (width - len)
    val (left, right) = (m div 2, m div 2 + m mod 2)
  in
    if width < len then substring (s, left, len - left - right)
    else (padLeft #" " (left + right + len) o padRight #" " (right + len)) s
  end