Transforming a MulSet data type to an array using Haskell

MultSet.hs

data MultSet = Nil | MS Int Int (MultSet)
    deriving Show
	
ins k (Nil) = MS k 1 (Nil)
ins k (MS e n ms)
    | k == e = MS e (n+1) ms
    | otherwise = MS e n (ins k ms)
	
ltm [] = Nil
ltm (x:xs) = ins x (ltm xs)

sublist _ _ [] = []
sublist k i (x:xs)
    | mod i k == 0 = x : ds
    | otherwise = ds
	where ds = sublist k (i+1) xs