Passage: The International Match Calendar, a key topic of discussions, makes the release of national team players compulsory for clubs on the dates it highlights. The 2014–18 International Match Calendar is based on a concrete proposal put forward by ECA, and the efforts of a dedicated working group comprising representatives from ECA, European Leagues, FIFPro, and UEFA. The working group's recommendation, acknowledged by FIFA, offers a more balanced system of nine double-headers over two years with no single friendly matches and is beneficial for both clubs and national associations.Insurance for Players' Salaries

Question: What groups laid the groundwork for the International Match Calendar?
ECA, European Leagues, FIFPro, and UEFA