$\forall$${\it es}$:event\_system\{i:l\}, $e$,${\it e'}$,${\it ev}$:es{-}E(${\it es}$).
\\[0ex](${\it ev}$ $\in$ [$e$, ${\it e'}$]) $\Leftarrow\!\Rightarrow$ (es{-}le(${\it es}$; $e$; ${\it ev}$) $\wedge$ es{-}le(${\it es}$; ${\it ev}$; ${\it e'}$))