public void AddElementToArray()
        {
            // Arrange

            var document = new XmlDocument(@"<xml>
<a>
  <x>1</x>
</a>
<b>2</b>
<c>3</c>
<d />
</xml>");


            // Act

            document.AddElementToArray(new[] { "xml", "d" }, "<val>1</val>");
            document.AddElementToArray(new[] { "xml", "d" }, "<val>2</val>");
            document.AddElementToArray(new[] { "xml", "f" }, "<x>1</x>");

            var result = document.ToString();


            // Assert

            Assert.Equal(@"<xml>
  <a>
    <x>1</x>
  </a>
  <b>2</b>
  <c>3</c>
  <d>
    <val>1</val>
    <val>2</val>
  </d>
  <f>
    <x>1</x>
  </f>
</xml>", result, ignoreCase: true, ignoreLineEndingDifferences: true, ignoreWhiteSpaceDifferences: true);
        }