public void MutateAtTest()
{
var document = new CsvDocument(new string[] { "name", "age" });
document.Write("Light", 18);
document.Write("Misa", 20);
document.MutateAt(0, record => record.Update("name", "Light Yagami"));
Assert.AreEqual("Light Yagami,18", document[0].ToString());
}