$\forall$${\it es}$:ES, $d$, $b$, $a$, $c$:E. $a$ $\leq$ $b$  $\Rightarrow$ $c$ $\leq$ $d$  $\Rightarrow$ [$a$, $b$] $=$ [$c$, $d$] $\in$ E List $\Rightarrow$ \{$a$ $=$ $c$ \& $b$ $=$ $d$\}