[1] psgi-specs/Extensions.pod at master · plack/psgi-specs () <https://github.com/plack/psgi-specs/blob/master/PSGI/Extensions.pod>
[2] Feersum - search.cpan.org () <http://search.cpan.org/dist/Feersum/lib/Feersum.pm#PSGI_extensions>
[3] Plack::App::WebSocket - search.cpan.org () <http://search.cpan.org/dist/Plack-App-WebSocket/lib/Plack/App/WebSocket.pm>
[4] uwsgi/psgi_plugin.c at master · unbit/uwsgi () <https://github.com/unbit/uwsgi/blob/master/plugins/psgi/psgi_plugin.c>
[5] PSGIアプリでWebSocketを使う場合、responderはどうすべきか? - DebugIto’s diary () <http://d.hatena.ne.jp/debug-ito/20131110/1384067449>
[6] P6SGI: More of a Journey than a Destination | Sterling Hanenkamp [blogs.perl.org] () <http://blogs.perl.org/users/sterling_hanenkamp/2015/09/p6sgi-more-of-a-journey-than-a-destination.html>