IsField($r$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$0$r$ $\neq$ 1$r$ $\in$ $\mid$$r$$\mid$  \& ($\forall$$u$:$\mid$$r$$\mid$. $u$ $\neq$ 0$r$ $\in$ $\mid$$r$$\mid$  $\Rightarrow$ $u$ $\mid$ 1$r$ in $r$)