code: def body(self, frame):
          
          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
docs: Creates the dialog body. Returns the widget that should have
              initial focus.