\begin{tabbing}
f{-}rel\{\$z,\$wanted\}(${\it es}$;$L$;$e_{1}$;$e_{2}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=($\neg$(es{-}loc(${\it es}$; $e_{2}$) = es{-}loc(${\it es}$; $e_{1}$) $\in$ Id))\+
\\[0ex]c$\wedge$ es{-}le(${\it es}$;f{-}wanted\{\$z,\$wanted\}(${\it es}$;$e_{1}$;es{-}loc(${\it es}$; $e_{2}$));$e_{2}$)
\-
\end{tabbing}