sys{-}valid\{i:l\}(${\it es}$; ${\it Config}$; ${\it Cmd}$; ${\it Sys}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$(${\it Sys}$$\mid$valid{-}sys{-}dcdr\{i:l\}(${\it es}$; ${\it Config}$; ${\it Cmd}$; ${\it Sys}$))