F-Sharp transposition function

let rec separate = function
| [] -> ([],[])
| []::xs -> ([],[]) 
| (x::xs)::ys   ->  let (M,N) = separate ys
                    (x::M, xs::N)

let rec trans = function
| [] ->  []
| x::xs ->  let (q, w) = separate (x::xs) in
            if List.length(q) = 0 then [] else [q] @ trans w