doc:   [
    DocGroup({"tag": "h2"}, [
        DocChars("12")
    ]),
    DocGroup({"tag": "p"}, [
        DocChars("1"),
    ]),
]

a_del: [
    DelGroup([
        DelSkip(2)
    ]),
    DelWithGroup([
        DelChars(1),
    ]),
]
a_add: [
    AddGroup({"tag": "h3"}, [
        AddGroup({"tag": "caret", "client": "d"}, []),
    ]),
    AddGroup({"tag": "h3"}, [
        AddSkip(2),
    ]),
    AddWithGroup([
        AddChars("6"),
    ]),
]

b_del: [
    DelGroup([
        DelSkip(2),
    ]),
    DelGroup([
        DelSkip(1)
    ])
]
b_add: [
    AddGroup({"tag": "h2"}, [
        AddSkip(3)
    ])
]