code: def _find_collections(root):
    
    packages = []
    for dirname, subdirs, files in os.walk(root):
      for filename in files:
        if filename.endswith('.xml'):
          xmlfile = os.path.join(dirname, filename)
          yield ElementTree.parse(xmlfile).getroot()
docs: Helper for ``build_index()``: Yield a list of ElementTree.Element
    objects, each holding the xml for a single package collection.