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$\vdash$  \=$\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)
\-
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