Append to XML structure in python

I would like to change/add a custom subelement to an xml which was generated by my script.

The top element is AAA:

top = Element('AAA')

The collected_lines looks like this:

[['TY', ' RPRT'], ['A1', ' Peter'], ['T3', ' Something'], ['ER', ' ']]

Then I enumerate all lines one-by-one and create a SubElement for top:

for line in enumerate(collected_lines):
 child = SubElement(top, line[0])
 child.text = line[1]

Output:

<?xml version="1.0" ?>
<AAA>
  <TY> RPRT</TY>
  <A1> Peter</A1>
  <T3> Something</T3>
  <ER> </ER>
</AAA>

And I would like to add <ART> element to the top element and then print the xml like this:

<?xml version="1.0" ?>
<AAA>
  <ART>
  <TY> RPRT</TY>
  <A1> Peter</A1>
  <T3> Something</T3>
  <ER> </ER>
  </ART>
</AAA>

I'm tried to do it with an if statemant. Like:

if "TY" in line:
 "append somehow before TY element, <ART>"
if "ER" in line:
 "append somehow after ER element, </ART>"

Is there a simple way to solve this?