$\forall$$r$:Rng, $b$, ${\it b'}$:$\mathbb{B}$, $p$:$\mid$$r$$\mid$. (when $b$. when ${\it b'}$. $p$) $=$ (when $b$ $\wedge_{2}$ ${\it b'}$. $p$) $\in$ $\mid$$r$$\mid$