def METHOD_NAME():
    lines = list(
        split_lines([("class:a", "line1"), ("class:b", "line2\nline3\nline4")])
    )
    assert lines == [
        [("class:a", "line1"), ("class:b", "line2")],
        [("class:b", "line3")],
        [("class:b", "line4")],
    ]