In international treaty negotiations, sovereign countries voluntarily decide whether to participate in an agreement. Examples are abundant. A prominent one is climate negotiation. As we reviewed in Subsection 6.3.1, developed countries participated in the Kyoto Protocol, and developing ones did not. Almost all countries participated in the Paris Agreement. Many international treaties have an ‘open access’ rule in that countries are free to participate. They should approve each other’s memberships. In contrast, the proposal–response game in Part II had a ‘closed access’ rule in that players could join a coalition only when a proposer invited them and they accepted the invitation.