How to modify the text of nested elements in xml file using python?

Currently I’m working on a corpus/dataset. It’s in xml format as you can see the picture below. I’m facing a problem. I want to access all ‘ne’ elements one by one as shown in below picture. Then I want to access the text of the ‘W’ elements which are inside the ‘ne’ elements. Then I want to concatenate thy symbols ‘SDi’ and ‘EDi’ with the text of these ‘W’ elements. ‘i’ can take any positive whole number starting from 1. In the case of ‘SDi’ I need only the text of first ‘W’ element that is inside the ‘ne’ element. In the case of ‘EDi’ I need only the text of last ‘W’ element that is inside the ‘ne’ element. Currently I don't get anything as output after running the code. I think this is because of the fact that the element 'W' is never accessed. Moreover, i think that element 'W' is not accessed because it is a grandchild of element 'ne' therefore it can't be accessed directly rather it may be possible with the help its father node.

Note1: The number and names of sub elements inside ‘ne’ elements are not same.

Note2: Only those things are explained here which needed. You may find some other details in the coding/picture but ignore them.

I'm using Spyder (python 3.6) Any help would be appreciated.

A picture from the XML file I'm working on is given below:

Text version of XML file: Click here

Sample/Expected output image (below):

Coding I've done so far:

for i in range(len(List_of_root_nodes)):
true_false = True
current = List_of_root_nodes[i]
start_ID = current.PDante_ID
#print('start:', start_ID)  # For Testing
end_ID = None
number = str(i+1)  # This number will serve as i used with SD and ED that is (SDi and EDi)

discourse_starting_symbol = "SD" + number
discourse_ending_symbol = "ED" + number

while true_false:    
    if current.right_child is None:        
        end_ID = current.PDante_ID
        #print('end:', end_ID)  # For Testing
        true_false = False        
    else:        
        current = current.right_child

# Finding 'ne' element with id='start_ID'
ne_text = None
ne_id = None

for ne in myroot.iter('ne'):    
    ne_id = ne.get('id')

    # If ne_id matches with start_ID means the place where SDi is to be placed is found    
    if ne_id == start_ID:        
        for w in ne.iter('W'):            
            ne_text = str(w.text)            
            boundary_and_text = " " + str(discourse_starting_symbol) + " " + ne_text
            w.text = boundary_and_text
            break

    # If ne_id matches with end_ID means the place where EDi is to be placed is found

    # Some changes Required here: Here the 'EDi' will need to be placed after the last 'W' element.
    # So last 'W' element needs to be accessed
    if ne_id == end_ID:        
        for w in ne.iter('W'):            
            ne_text = str(w.text)            
            boundary_and_text = ne_text + " " + str(discourse_ending_symbol) + " "
            w.text = boundary_and_text
            break

Something like this (a.xml is the XML you have uploaded):

Note the code is not using any external library.

import xml.etree.ElementTree as ET

SD = 'SD'
ED = 'ED'

root = ET.parse('a.xml')

counter = 1

for ne in root.findall('.//ne'):
    w_lst = ne.findall('.//W')
    if w_lst:
        w_lst[0].text = '{}{} {}'.format(SD, counter, w_lst[0].text)
        if len(w_lst) > 1:
            w_lst[-1].text = '{} {}{}'.format(w_lst[-1].text, ED, counter)
        counter += 1
ET.dump(root)