let matrix m =                                   
  let remove_first_two = function                
  | (h1 :: h2 :: t) -> List.rev t                
  | _ -> raise (Invalid_argument "invalid list") 
  in                                             
  let remove_last = function                     
    | h :: t -> List.rev t                       
    |_ -> raise (Invalid_argument "invalid list")
  in remove_last (remove_first_two m)