es{-}seq(${\it es}$;$S$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$e_{1}$, $e_{2}$:E. $e_{1}$ before $e_{2}$ $\in$ $S$ $\Rightarrow$ $\neg$($e_{2}$ $<$ $e_{1}$)