type 'a node = {
  value: 'a;
  successors: ('a node) list;
  mutable tmp: exn
}

open Graph

exception None

let rec a = {
  value = "a";
  successors = [b];
  tmp = None
}
and b = {
  value = "b";
  successors = [a];
  tmp = None
};

exception Visited

let depth_first f n =
  let rec aux n =
    match n.tmp with
    | Visited -> ()
    | _ -> begin
        f n.value;
        n.tmp <- Visited;
        List.iter aux n.successors
    end
  in
  aux n

let _ =
  depth_first print_string a;
  print_newline ()