chain{-}order(${\it es}$;${\it Sys}$;${\it chain}$;$x$;$y$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\exists$$e$:es{-}E{-}interface(${\it es}$;${\it Sys}$). $x$ before $y$ $\in$ ${\it chain}$($e$) $\in$ Id