F#: generate all emirp number which digits form descent order

emirp.fs

(* 
F# solution of below URL
http://gushwell.ifdef.jp/etude/RisingEmirp.html
*)

let isPrime n =
    seq{yield 2;yield![3..2..int<<sqrt<|float n]}
    |> Seq.forall(fun d->n%d<>0)
let rec revds a n= if n=0 then a else revds(a*10+n%10)(n/10)
let upNums = List.fold(fun ns d->List.collect(fun n->[n;n*10+d])ns)[0][1..9]
let upEmirp = upNums |> List.filter(fun n-> n>9 && isPrime n && isPrime<|revds 0 n) |> List.sort

upEmirp |> List.iter(printfn"%d")
Console.ReadKey()|>ignore