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