More formally, the $n*{th}$ block proposed by a given validator contains values $(r_n, s_n)$ such that $\text{keccack256}(r_n) = s*{n-1}$. The one exception to this is the validator’s first block, the case where $n = 1$, since they have not previously committed to randomness yet. Here, the protocol instead requires that $r_1 = 1$.