docs: def from_xml(content_types_xml):
          """
          Return a new |_ContentTypeMap| instance populated with the contents
          of *content_types_xml*.
          """
          types_elm = parse_xml(content_types_xml)
          ct_map = _ContentTypeMap()
          for o in types_elm.overrides:
              ct_map._add_override(o.partname, o.content_type)
          for d in types_elm.defaults:
              ct_map._add_default(d.extension, d.content_type)
          return ct_map