$\forall$${\it the\_es}$:ES, ${\it e'}$:E, $n$:$\mathbb{N}$.
\\[0ex]$n$$<\parallel$before(${\it e'}$)$\parallel$ $\Rightarrow$ firstn($n$;before(${\it e'}$)) $=$ before(before(${\it e'}$)[$n$]) $\in$ E List