input: Write a function to parse an XML document and return a list of tag objects. Each tag object should contain the tag's id and a list of values from its subchildren. If no subchildren are present, the list of values should be empty. Provide a test case that demonstrates the function's usage and expected output.
  
  
  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'])]
  
  
  class Tag:
      def __init__(self, tag_id, values=None):
          self.tag_id = tag_id
          self.values = values if values is not None else []
      
      def __eq__(self, other):
          return self.tag_id == other.tag_id and self.values == other.values
output: def parse_xml(xml_string):
      import xml.etree.ElementTree as ET
      root = ET.fromstring(xml_string)
      tags = []
      for child in root:
          tag_id = child.attrib['id']
          values = [subchild.attrib['value'] for subchild in child]
          tags.append(Tag(tag_id, values))
      return tags