$\forall$${\it es}$:event\_system\{i:l\}, $e$,${\it e'}$:es{-}E(${\it es}$).
\\[0ex][$e$, ${\it e'}$] $\in$ (\{${\it ev}$:es{-}E(${\it es}$)$\mid$ loc(${\it ev}$) = loc(${\it e'}$) $\in$ Id\}  List)