strong\_safety($T$;${\it tr}$.$P$(${\it tr}$))
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$${\it tr}_{1}$:$T$ List, ${\it tr}_{2}$:$T$ List. sublist($T$; ${\it tr}_{1}$; ${\it tr}_{2}$) $\Rightarrow$ $P$(${\it tr}_{2}$) $\Rightarrow$ $P$(${\it tr}_{1}$)