\begin{tabbing}
cmd{-}history\=\{i:l\}\+
\\[0ex](${\it es}$; ${\it Config}$; ${\it Cmd}$; ${\it Sys}$; $e$)
\-\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$es{-}interface{-}history(${\it es}$;$\lambda$$x$.sys{-}cmds($x$)'sys{-}valid\=\{i:l\}\+
\\[0ex](${\it es}$; ${\it Config}$; ${\it Cmd}$; ${\it Sys}$);$e$)
\-
\end{tabbing}