strongwellfounded($T$; $x$,$y$.$R$($x$;$y$)) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\exists$$f$:($T$$\rightarrow\mathbb{N}$). $\forall$$x$:$T$, $y$:$T$. $R$($x$;$y$) $\Rightarrow$ $f$($x$)$<$$f$($y$)