input: from typing import List
  import xml.etree.ElementTree as ET
  
  class Tag:
      def __init__(self, tag_id):
          self.tag_id = tag_id
          self.values = []
      
      def add_value(self, value):
          self.values.append(value)
  
  def parse_xml(text: str) -> List[Tag]:
      """ Parse an XML document and return a list of Tag objects.
      Each Tag object should contain the id attribute of each child and 
      the value attribute of each subchild in that child.
  
      >>> parse_xml('''
      ... <xml>
      ...   <root>
      ...     <child1 id="1">
      ...       <subchild1 value="a"></subchild1>
      ...       <subchild2 value="b"></subchild2>
      ...     </child1>
      ...     <child2 id="2">
      ...       <subchild3 value="c"></subchild3>
      ...       <subchild4 value="d"></subchild4>
      ...     </child2>
      ...   </root>
      ... </xml>''')[0].tag_id
      '1'
      >>> parse_xml('''
      ... <xml>
      ...   <root>
      ...     <child1 id="1">
      ...       <subchild1 value="a"></subchild1>
      ...       <subchild2 value="b"></subchild2>
      ...     </child1>
      ...     <child2 id="2">
      ...       <subchild3 value="c"></subchild3>
      ...       <subchild4 value="d"></subchild4>
      ...     </child2>
      ...   </root>
      ... </xml>''')[1].values[0]
      'c'
      """
      tag_objects = []
  
      root = ET.fromstring(text)
      for child in root:
          tag = Tag(child.attrib["id"])
          for subchild in child:
              tag.add_value(subchild.attrib["value"])
          tag_objects.append(tag)
  
      return tag_objects
output:     tag_objects = []
  
      root = ET.fromstring(text)
      for child in root:
          tag = Tag(child.attrib["id"])
          for subchild in child:
              tag.add_value(subchild.attrib["value"])
          tag_objects.append(tag)
  
      return tag_objects