\begin{tabbing}
es{-}interval\=\{i:l\}\+
\\[0ex](${\it es}$; $e$; ${\it e'}$)
\-\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$filter($\lambda$${\it ev}$.es{-}ble\{i:l\}(${\it es}$; $e$; ${\it ev}$);es{-}before(${\it es}$; ${\it e'}$) @ (${\it e'}$.nil))
\end{tabbing}