\begin{tabbing}
es{-}sends{-}on(${\it es}$;$e$;$l$;${\it tg}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\exists$\=${\it e'}$:es{-}E(${\it es}$)\+
\\[0ex]((es{-}kind(${\it es}$; ${\it e'}$) = rcv($l$,${\it tg}$) $\in$ Knd) c$\wedge$ (es{-}sender(${\it es}$; ${\it e'}$) = $e$ $\in$ es{-}E(${\it es}$)))
\-
\end{tabbing}