rel{-}path{-}between($T$;$R$;$x$;$y$;$L$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$rel{-}path($R$;$L$) \& (0 $<$ $\parallel$$L$$\parallel$) \& $x$ = hd($L$) $\in$ $T$ \& $y$ = last($L$) $\in$ $T$