// кратко
let rec sec1 xs = 
    match xs with a::_::t | a::([] as t) -> a :: sec1 t | _ -> []
 
// быстро
let sec2 xs = 
    let rec loop acc = function 
                       | a::_::t | a::([] as t) -> loop (a::acc) t
                       | _ -> List.rev acc
    loop [] xs
 
// 
let sec3 xs = 
    xs |> List.mapi (fun i e -> if i % 2 = 0 then Some e else None) 
       |> List.choose id
 
open System.Linq
let sec4 (xs : 'a list) = 
    xs.Where(fun _ i -> i % 2 = 0) |> Seq.toList
 
 
 
let sp = [0..10]
sp |> printfn "%A" 
sec1 sp |> printfn "%A"
sec2 sp |> printfn "%A"
sec3 sp |> printfn "%A"
sec4 sp |> printfn "%A"
 

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