chain{-}order{-}le(${\it es}$;${\it Sys}$;${\it chain}$;$x$;$y$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$chain{-}order(${\it es}$;${\it Sys}$;${\it chain}$;$x$;$y$) $\vee$ ($x$ = $y$ $\in$ Id)