\begin{tabbing}
es{-}send{-}atom(${\it es}$;$e$;$a$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=$\forall$${\it e'}$:es{-}E(${\it es}$).\+
\\[0ex]($\uparrow$es{-}isrcv(${\it es}$; ${\it e'}$))
\\[0ex]$\Rightarrow$ (es{-}sender(${\it es}$; ${\it e'}$) = $e$ $\in$ es{-}E(${\it es}$))
\\[0ex]$\Rightarrow$ free{-}from{-}atom\{1\}(es{-}valtype(${\it es}$; ${\it e'}$);es{-}val(${\it es}$; ${\it e'}$);$a$)
\-
\end{tabbing}