$\forall$${\it es}$:ES, $X$, $Y$:AbsInterface(Top). E([$X$?$Y$]) $\subseteq$r \{$e$:E$\mid$ ($\uparrow$($e$ $\in_{b}$ $X$)) $\vee$ ($\uparrow$($e$ $\in_{b}$ $Y$))\}