FParsec.FF.Sequence.fs

// Parser<('a -> 'b), 'u> -> Parser<'a, 'u> -> Parser<'b, 'u>
let apply fp xp =
    pipe2 fp xp (fun f x -> f x)

// ('a -> Parser<'b, 'u>) -> 'a list -> Parser<'b list, 'u>
let traverse f list =
    let (<*>) = apply
    let retn = preturn
    let cons head tail = head :: tail
     
    let init = retn []
    let folder head tail =
        retn cons <*> (f head) <*> tail

    List.foldBack folder list init

// Parser<'a, 'u> list -> Parser<'a list, 'u>
let sequence x = traverse id x