$\forall$${\it es}$:ES, ${\it Config}$:AbsInterface(chain\_config()).
\\[0ex]tail{-}dcdr\{i:l\}(${\it es}$;${\it Config}$) $\in$ $e$:E$\rightarrow$Dec($\exists$$c$$<$$e$.($\uparrow$($c$ $\in_{b}$ ${\it Config}$)) \& ($\uparrow$cctail?(${\it Config}$($c$))))