anchor.diff:                                     {'min-expires', <<"213">>}, {'mime-version', <<"1.0">>},
                                      {'organization', <<"Boxes by Bob">>}, {'priority', <<"non-urgent">>},
                                      {'proxy-authenticate', <<"Digest realm=\"atlanta.com\"">>}, {'proxy-authorization', <<"Digest username=\"Alice\"">>},
  +                                   {'proxy-require', <<"foo">>},
   
   
                                      {'content-length', <<"5">>}, {'via', <<"SIP/2.0/UDP localhost">>},
  
positive.code:     % same as Authorization header
      process(f, 'authorization', Auth);
  
  
  %% .....
  process(pn, <<"v">>, _Ignore) -> 'via';
  
negative.code:     parse_list('require', Ext, Rest);
  
  process(f, 'require', Ext) ->
      sip_binary:any_to_binary(Ext);
  
  %% ...
  