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