How to insert new node in existing xml file using TCL(TDom)

I have an existing XML document with more number of nodes and I want to insert a new node, but at a certain position.

The document looks something like:

<root>   
    <a>...</a&开发者_开发技巧gt;
    <c>...</c>
    <e>...</e>
</root> 

... can be considered as xml tags a.../a, c.../c, e.../e. (formatting issue)

The new nodes should be inserted in alphabetical order in between the nodes, resulting in:

<root>
    <>
    new node
    <>
    <>
    new node
    <>
    <>
    <>
    new node

How can I use XPath in TCL to find the existing node and insert new node before or after it.

I also want to preserve the order, since the existing tags in XML document are in alphabetical order.

At present I am using tdom package.

Does anyone have an idea on how to insert such a node?

---

If you've got this in a file, demo.xml:

<root>
    <a>123</a>
    <c>345</c>
    <e>567</e>
</root>

And want to go to this (modulo whitespace):

<root>
    <a>123</a>
    <b>234</b>
    <c>345</c>
    <d>456</d>
    <e>567</e>
</root>

Then this is the script to do it:

# Read the file into a DOM tree
package require tdom
set filename "demo.xml"
set f [open $filename]
set doc [dom parse [read $f]]
close $f

# Insert the nodes:
set container [$doc selectNodes /root]

set insertPoint [$container selectNodes a]
set toAdd [$doc createElement b]
$toAdd appendChild [$doc createTextNode "234"]
$container insertAfter $insertPoint $toAdd

set insertPoint [$container selectNodes c]
set toAdd [$doc createElement d]
$toAdd appendChild [$doc createTextNode "456"]
$container insertAfter $insertPoint $toAdd

# Write back out
set f [open $filename w]
puts $f [$doc asXML -indent 4]
close $f

---

I'm pretty sure that the tutorial for tdom on the Tcl wiki answers all of your questions. There is some additional information on Xpath on the wiki as well.