$\forall$${\it es}$:ES, $e_{1}$:E, $e_{2}$:\{$e$:E$\mid$ loc($e$) = loc($e_{1}$) $\in$ Id\} . $\exists$$e$$\in$($e_{1}$,$e_{2}$].True $\Leftarrow\!\Rightarrow$ ($e_{1}$ $<$loc $e_{2}$)