let containsAs (s : string) =
    String.filter (fun c -> c = 'a') s |> String.length

let numOfStringsContainingAs (str : string []) =
    str
    |> Array.map (fun s -> containsAs(s))
    |> Array.sum

printfn "%A" (numOfStringsContainingAs [|"ciao";"come";"va"|])
printfn "%A" (numOfStringsContainingAs [|"ciaaao";"come";"va"|])

let numeroPippo x = if x%3 = 1 then Some(x) else None
let parolaPippo x = if x = "pippo" then Some(x) else None

let isPippo (x : option<'a>) =
    match x with
    | Some(x) -> true
    | None -> false

printfn "%b" (isPippo(numOfStringsContainingAs [|"a";"a";"a";"a"|] |> numeroPippo))
printfn "%b" (isPippo(numOfStringsContainingAs [|"a";"a";"a"|] |> numeroPippo))
printfn "%b" (isPippo(parolaPippo("pipo")))
printfn "%b" (isPippo(parolaPippo("pippo")))

let s = Seq.initInfinite (fun s -> s*s)
Seq.toList (Seq.take 10 s)