let rec _trace2 (sg : SectionGraph) (starting : Vertex) (section : Section)
                    acc successp filterp stopp =
        match section.elements
                |> List.filter (fun el -> vEq el starting |> not)
                |> List.tryFind (fun el -> successp section el) with
        | Some (el) ->
            [((section, Some(el)) :: acc)]
        | None ->
            let outs =
                match acc with
                | (s,_) :: _ -> sg.outSectionsWrs (section, s)
                | _ -> sg.outSections (section)
                        |> List.filter
                            (fun (s : Section) ->
                                not (s.have_element (fun a -> vEq a starting)))
            outs
                |> List.filter
                    (fun (s : Section) ->
                        filterp s
                            && not(s.elements |> List.exists (fun e -> vEq e starting))
                                && not(acc |> List.exists (fun (ex,_) -> ex.id = s.id)))
                |> List.collect
                    (fun s ->
                        _trace2 sg starting s ((section, None) :: acc)
                            successp filterp stopp)

member x.trace2 id successp filterp stopp =
        let start_v = x.vertices |> List.find (fun el -> el.Id.OriginalId = id)
        let sections =
            x.sections
                |> List.filter (fun s -> s.have_element (fun el -> vEq el start_v))
                |> List.filter
                    (fun s ->
                        let el = s.outermostElement start_v
                        GraphExtensions.in_one_coursep start_v el)
        sections
            |> List.collect
                (fun s -> _trace2 x start_v s [] successp filterp stopp)