strong\_safety($T$;${\it tr}$.$P$(${\it tr}$)) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$${\it tr}_{1}$, ${\it tr}_{2}$:$T$ List. ${\it tr}_{1}$ $\subseteq$ ${\it tr}_{2}$ $\Rightarrow$ $P$(${\it tr}_{2}$) $\Rightarrow$ $P$(${\it tr}_{1}$)