$\forall$${\it the\_es}$:event\_system\{i:l\}, $e$,${\it e'}$:es{-}E(${\it the\_es}$).
\\[0ex]es{-}le(${\it the\_es}$; $e$; ${\it e'}$)
\\[0ex]$\Leftarrow\!\Rightarrow$ ((($\neg$($\uparrow$es{-}first(${\it the\_es}$; ${\it e'}$))) c$\wedge$ es{-}le(${\it the\_es}$; $e$; es{-}pred(${\it the\_es}$; ${\it e'}$))) $\vee$ ($e$ = ${\it e'}$))