Pairing operator in F#

gistfile1.fs

// 'a seq -> ('a * 'a) seq
let pair (series: 'a seq) = seq {
    use enum = series.GetEnumerator()
    if enum.MoveNext() then
        let first = ref (Some enum.Current)

        while enum.MoveNext() do
            if Option.isSome !first then
                yield (Option.get !first), enum.Current
                first := None
            else
                first := Some enum.Current

        if Option.isSome !first then
            let msg = sprintf "Dangling element: %A" (Option.get !first)
            failwith msg
}