\begin{tabbing}
$\vdash$  \=$\forall$$T$:Type, $x$, $y$, $u$:$T$, $L$:($T$ List).\+
\\[0ex]adjacent($T$;[$u$ / $L$];$x$;$y$) $\Leftarrow\!\Rightarrow$ ((0 $<$ $\parallel$$L$$\parallel$) \& (($x$ = $u$ \& $y$ = hd($L$)) $\vee$ adjacent($T$;$L$;$x$;$y$)))
\-
\end{tabbing}