-- 30C3: https://events.ccc.de/congress/2013/Fahrplan/schedule.xml
-- DebConf (2013, pentbarf): https://penta.debconf.org/dc13_schedule/schedule.en.xml
-- DebConf (2014, summit): https://summit.debconf.org/debconf14.xml
-- FOSDEM: http://fosdem.org/schedule/xml
-- FrOSCon (2014): http://programm.froscon.org/2014/schedule.xml
-- Grazer Linuxtage (2014): http://glt14-programm.linuxtage.at/schedule.de.xml
+https://github.com/voc/schedule/blob/master/validator/xsd/schedule.xml.xsd
+
+
+Tested PentaBarf (or frab, or Summit/Wafer with patches, or pretalx' XML
+export) instances:
+
+- 37C3 (pretalx): https://fahrplan.events.ccc.de/congress/2023/Fahrplan/schedule.xml
+- RC3: https://fahrplan.events.ccc.de/rc3/2020/Fahrplan/schedule.xml
+- DebConf (2015, summit): https://summit.debconf.org/debconf15.xml
+- DebConf (2021, wafer): https://debconf21.debconf.org/schedule/pentabarf.xml
+- FOSDEM: https://fosdem.org/schedule/xml
+- FrOSCon (2020): https://programm.froscon.org/2020/schedule.xml
+- Grazer Linuxtage (2018): https://glt18-programm.linuxtage.at/schedule.xml