input: Write a function to parse an XML document and return a list of tag objects. Each tag object should have an ID and a list of values corresponding to subchildren. The function should take an XML string as input and return a list of Tag objects. If the XML is empty, the function should return an empty list.
  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('<xml><root><child id="1"><subchild value="x"></subchild></child></root></xml>')==[Tag('1', ['x'])]
  assert parse_xml('')==[]
output: def parse_xml(xml_string):
      if not xml_string:
          return []
      
      from xml.etree import ElementTree as ET
      
      root = ET.fromstring(xml_string)
      tags = []
      
      for child in root.findall('.//child'):
          tag_id = child.get('id')
          values = [subchild.get('value') for subchild in child.findall('.//subchild')]
          tags.append(Tag(tag_id, values))
      
      return tags