$\forall$${\it es}$:ES, ${\it e'}$:E, $P$:(\{$e$:E$\mid$ loc($e$) = loc(${\it e'}$) $\in$ Id\} $\rightarrow\mathbb{P}$).
\\[0ex]($\forall$$e$$<$${\it e'}$. $P$($e$)) $\Leftarrow\!\Rightarrow$ (($\neg$($\uparrow$first(${\it e'}$))) $\Rightarrow$ ($P$(pred(${\it e'}$)) \& ($\forall$$e$$<$pred(${\it e'}$). $P$($e$))))