\begin{tabbing}
$\forall$\=${\it es}$:event\_system\{i:l\}, $e_{1}$:es{-}E(${\it es}$), $e_{2}$:\{$e$:es{-}E(${\it es}$)$\mid$ loc($e$) = loc($e_{1}$) $\in$ Id\} ,\+
\\[0ex]$p$,$q$:(\=\{$e$:es{-}E(${\it es}$)$\mid$ loc($e$) = loc($e_{1}$) $\in$ Id\} $\rightarrow$\{$e$:es{-}E(${\it es}$)$\mid$ loc($e$) = loc($e_{1}$) $\in$ Id\} $\rightarrow$\+
\\[0ex]prop\{i:l\}). [$e_{1}$;$e_{2}$]$\sim$([$a$,$b$].$p$($a$,$b$))$\ast$[$a$,$b$].$q$($a$,$b$) $\in$ prop\{i:l\}
\-\-
\end{tabbing}