\begin{tabbing}
sub{-}es{-}sender(${\it es}$;${\it dom}$;$e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$if ${\it dom}$(sender($e$)) then sender($e$) else sub{-}es{-}sender(${\it es}$;${\it dom}$;sender($e$)) fi 
\\[0ex]
\\[0ex]{\em clarification:}
\\[0ex]
\\[0ex]sub{-}es{-}sender(${\it es}$;${\it dom}$;$e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if ${\it dom}$(es{-}sender(${\it es}$; $e$))\+
\\[0ex]then es{-}sender(${\it es}$; $e$)
\\[0ex]else sub{-}es{-}sender(${\it es}$;${\it dom}$;es{-}sender(${\it es}$; $e$))
\\[0ex]fi 
\-\\[0ex]\emph{(recursive)}
\end{tabbing}