fifo{-}antecedent(${\it es}$;${\it Sys}$;$f$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$e$, ${\it e'}$:E(${\it Sys}$). (loc($e$) = loc(${\it e'}$)) $\Rightarrow$ $f$($e$) $\leq$loc $f$(${\it e'}$)  $\Rightarrow$ $e$ $\leq$loc ${\it e'}$