{"ast_node_type":"variable","tokens":[{"end":57,"kind":"keyword","start":0,"text":"let"}]}
    let (t, r) = parse_heading(buf1.as_bytes()).unwrap();
    let buf1 = "# Top level\nrest";
    assert_eq!(t, MdTree::Heading(1, vec![MdTree::PlainText("Top level")].into()));