$r${-}Prime($u$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$($\neg$$u$ $\mid$ 1 in $r$) \& ($\forall$$v$, $w$:$\mid$$r$$\mid$. $u$ $\mid$ $v$ $\ast$ $w$ in $r$ $\Rightarrow$ ($u$ $\mid$ $v$ in $r$ $\vee$ $u$ $\mid$ $w$ in $r$))