$\forall$$T$:Type, $L$:($T$ List).
\\[0ex]($\neg$($\uparrow$null($L$))) $\Rightarrow$ no\_repeats($T$;$L$) $\Rightarrow$ ($\forall$$x$:$T$. last($L$) before $x$ $\in$ $L$ $\Leftarrow\!\Rightarrow$ False)