\begin{tabbing}
$\forall$$T$:Type, ${\it eq}$:EqDecider($T$), ${\it es}$:event\_system\{i:l\}, $x$:Id, $e$:\{\=$e$:es{-}E(${\it es}$)$\mid$ \+
\\[0ex]es{-}dtype(${\it es}$; loc($e$); $x$; $T$)\} .
\-\\[0ex]change{-}to($x$;$e$) $\in$ (?es{-}E(${\it es}$))
\end{tabbing}