method private select =
    let selected s =
      match selected with
        | Some (child,_) when child==s -> true
        | _ -> false
    in try
      Some (third (List.find
                   (fun (d,single,s) ->
                      (* Check single constraints *)
                      (if selected s then
                         not single
                       else
                         true) &&
                      (satisfied d) && s.source#is_ready)
                   children))
    with
      | Not_found -> None