In order for an initial assumption to be an expansion, it must be that a subformula F {\displaystyle F} is entailed if and only if ◻ F {\displaystyle \Box F} has been initially assumed true.
为了使初始假定是一个展开，一个子公式 F {\displaystyle F} 被蕴涵是必须的当且仅当 ◻ F {\displaystyle \Box F} 已经被初始假定为真。