\begin{tabbing}
$\forall$\=${\it es}$:ES, ${\it Config}$:AbsInterface(chain\_config()), ${\it Cmd}$:Type, ${\it Sys}$:AbsInterface(chain\_sys(${\it Cmd}$)),\+
\\[0ex]$e$:E.
\-\\[0ex]Dec(($\uparrow$($e$ $\in_{b}$ ${\it Sys}$)) c$\wedge$ valid{-}sys(${\it es}$;${\it Config}$;${\it Sys}$;$e$))
\end{tabbing}