let s = "abbc"

let b = "babcabbacaabcbabcacbb"

​

let find (s : string) (b : string) =

  let sLen = s.Length

  let left = s |> Seq.sort |> Seq.toArray

  b 

  |> Seq.windowed sLen

  |> Seq.mapi (fun idx window -> 

    let right = window |> Seq.sort |> Seq.toArray

    if left = right then Some idx else None)

  |> Seq.choose id

  |> Seq.toArray

find s b |> printfn "%A"