input: Write a function to parse an XML document and return a list of tag objects, where each tag object contains an ID and a list of its subchild values. If the input XML is empty, 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></root></xml>\") == []
  assert parse_xml(\"\") == []
output: 
  def parse_xml(xml_str):
      if not xml_str:
          return []
      
      from xml.etree import ElementTree as ET
      
      root = ET.fromstring(xml_str)
      tags = []
      
      for child in root.findall('.//child1') + root.findall('.//child2'):
          tag_id = child.attrib['id']
          subchild_values = [subchild.attrib['value'] for subchild in child]
          tags.append(Tag(tag_id, subchild_values))
      
      return tags
  