input: Write a function to parse an XML document and return a list of tag objects, where each tag object has an id and a list of values from its subchildren. If the XML document is empty, return an empty list.
  
  def parse_xml(text):
      if not text:
          return []
      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
  
  class Tag:
      def __init__(self, tag_id):
          self.tag_id = tag_id
          self.values = []
      
      def add_value(self, value):
          self.values.append(value)
  
  assert 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>
  ''') == [Tag('1', ['a', 'b']), Tag('2', ['c', 'd'])]
  
  assert parse_xml('') == []
output: def parse_xml(text):
      if not text:
          return []
      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