Let the set of worlds in a Kripke frame be $W = \{w_1, w_2, w_3\}$, and let the accessibility relation be $R = \{(w_1, w_2), (w_2, w_3), (w_3, w_1)\}$. The valuation of a formula $P$ is $V(P) = \{w_1, w_2\}$. Recall that $w \in V(\Box \phi)$ if and only if for all $u \in W$ such that $(w, u) \in R$, we have $u \in V(\phi)$. What is the set of worlds where the modal formula $\Box(\Box P \implies P)$ is true?