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did{-}forward(${\it es}$; ${\it Sys}$; $f$; $e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\exists$\=$a$:es{-}E{-}interface(${\it es}$;${\it Sys}$)\+
\\[0ex]($f$($a$) = $e$ $\in$ es{-}E(${\it es}$) \& ($\neg$(es{-}loc(${\it es}$; ($f$($a$))) = es{-}loc(${\it es}$; $a$) $\in$ Id)))
\-
\end{tabbing}