let groupsOf (size: int) = Seq.map snd << Seq.filter (fst >> flip (%) size >> (=) 0) << Seq.mapi tuple2 << Seq.windowed size
  let groupsOfAtMost (size: int) (s: seq<_>) =
    seq {
      let en = s.GetEnumerator ()
      let more = ref true
      while !more do
        let group =
          [
            let i = ref 0
            while !i < size && en.MoveNext () do
              yield en.Current
              i := !i + 1
          ]
        if List.isEmpty group then
          more := false
        else
          yield group
    }