\begin{tabbing}
cr{-}output\=\{i:l\}\+
\\[0ex](${\it es}$; ${\it Cmd}$; ${\it Sys}$; ${\it Config}$)
\-\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$(sys{-}valid\=\{i:l\}\+
\\[0ex](${\it es}$; ${\it Config}$; ${\it Cmd}$; ${\it Sys}$)$\mid$tail{-}dcdr\=\{i:l\}\+
\\[0ex](${\it es}$; ${\it Config}$))
\-\-
\end{tabbing}