let rec ArrayToFlatArray arr : ('a [] * int64 []) =
     let checkRegularity arrs : unit =
        if Array.length arrs = 0
        then failwith "Empty array"

        let head = Array.head arrs
        if not <| Array.forall (fun l -> l = head) arrs
        then failwith "Irregular array"

     let subType = (Array.head arr).GetType()
     if subType.IsArray
     then
        let a = Array.map ArrayToFlatArray arr
        let (subarrs, lens) = Array.unzip a
        checkRegularity lens
        let subarrs' = Array.concat subarrs
        let lens' = Array.head lens
        let len_subarrs = int64 <| Array.length arr
        let lens_out = Array.append [|len_subarrs|] lens'
        (subarrs', lens_out)
      else
        (arr, [|int64 <| Array.length arr|])