How to put a GridLayout inside a Layout?

Question

Python

There is this kv markup code:

<Layout>:
  id: main_layout
  orientation: 'vertical'
  Button:
    text: '1'
  Button:
    text: '2'
  Button:
    text:'3

How can I insert a GridLayout instead of the second button. If you stupidly replace the button with layout, it gives an error. how is it done humanly?