\begin{tabbing}
$\forall$$T$:Type, $R$:($T$$\rightarrow$$T$$\rightarrow\mathbb{P}$), $L$:($T$ List), $x$, $y$, $z$:$T$.
\\[0ex]rel{-}path{-}between($T$;$R$;$x$;$y$;[$z$ / $L$])
\\[0ex]$\Leftarrow\!\Rightarrow$ \=($x$ = $z$\+
\\[0ex]\& (($\uparrow$null($L$)) $\Rightarrow$ ($y$ = $z$))
\\[0ex]\& (($\neg$($\uparrow$null($L$))) $\Rightarrow$ (($x$ $R$ hd($L$)) \& rel{-}path{-}between($T$;$R$;hd($L$);$y$;$L$))))
\-
\end{tabbing}