Creates the dialog body. Returns the widget that should have
            initial focus.

def body(self, frame):
        """ Creates the dialog body. Returns the widget that should have
            initial focus.
        """
        master = Frame(self)
        master.pack(padx=5, pady=0, expand=1, fill=BOTH)

        title = Label(master, text="Buses")
        title.pack(side=TOP)

        bus_lb = self.bus_lb = Listbox(master, selectmode=SINGLE, width=10)
        bus_lb.pack(side=LEFT)

        for bus in self.case.buses:
            bus_lb.insert(END, bus.name)

        bus_lb.bind("<<ListboxSelect>>", self.on_bus)

        self.bus_params = BusProperties(master)

        return bus_lb