  import copy
  from collections import OrderedDict
  
  
  class OrderedSet:
      """
      A set which keeps the ordering of the inserted items.
      Currently backs onto OrderedDict.
 
</s> remove     DictWrapper, ImmutableList, MultiValueDict, MultiValueDictKeyError,
    OrderedSet,
 </s> add     CaseInsensitiveMapping, DictWrapper, ImmutableList, MultiValueDict,
    MultiValueDictKeyError, OrderedSet, </s> remove from django.http.request import split_domain_port
 </s> add from django.http.request import HttpHeaders, split_domain_port </s> remove from django.utils.datastructures import ImmutableList, MultiValueDict
 </s> add from django.utils.datastructures import (
    CaseInsensitiveMapping, ImmutableList, MultiValueDict,
) </s> remove * ...
 </s> add * Added :attr:`.HttpRequest.headers` to allow simple access to a request's
  headers.
from collections.abc import Mapping
  import copy
  from collections import OrderedDict
from collections.abc import Mapping
  
  
  class OrderedSet:
      """
      A set which keeps the ordering of the inserted items.
      Currently backs onto OrderedDict.
keep add keep keep keep keep keep keep
 Fixed #20147 -- Added HttpRequest.headers. 
django/utils/datastructures.py