content: sub initScreenStack()
      m.screenStack = []
  end sub
  
  sub showScreen(node as object)
      m.top.appendChild(node)
      node.visible = true
      prev = m.screenStack.peek()
      if prev <> invalid
          prev.visible = false
      end if
      node.setFocus(true)
      m.screenStack.push(node)
  end sub
  
  sub closeScreen()
      last = m.screenStack.pop()
      m.top.removeChild(last)
      prev = m.screenStack.peek()
      if prev <> invalid
          prev.visible = true
          prev.setFocus(true)
      end if
  end sub