\begin{tabbing}
poss{-}le\=\{i:l\}\+
\\[0ex]($e_{1}$; $e_{2}$)
\-\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$1of($e_{1}$) $=$ 1of($e_{2}$) $\in$ ES\{i\} \& es{-}le(1of($e_{1}$);1of(2of($e_{1}$));1of(2of($e_{2}$)))
\end{tabbing}