diff.ml

let diff xs ys =                                           
  let rec internal xs ys ans =                             
    match xs with                                          
      | hd::tl ->                                          
          if List.mem hd ys then internal tl ys ans        
          else internal tl ys (ans @ [hd])                 
      | _ -> ans                                           
  in                                                       
  internal xs ys [];;                                      
                                                           
diff [1;2;3;4;5;6] [2;4;6];;