I want to modify all the text nodes using some functions in C#.
I want to insert another xml subtree created from some string.

For example, I want to change this

```
<root>
this is a test
</root>

```

to

```
<root>
this is <subtree>another</subtree> test
</root>

```

I have this piece of code, but it inserts text node, I want to create xml subtree and insert that instead of plain text node.

```
List<XText> textNodes = element.DescendantNodes().OfType<XText>().ToList();
foreach (XText textNode in textNodes)
{
    String node = System.Text.RegularExpressions.Regex.Replace(textNode.Value, "a", "<subtree>another</subtree>");
    textNode.ReplaceWith(new XText(node));
} 

```