$\forall$$T$:Type, ${\it as}$, ${\it bs}$:$T$ List, $x$:$T$.
\\[0ex]($\neg$($x$ $\in$ ${\it bs}$) $\Rightarrow$ (agree\_on\_common($T$;$x$.${\it as}$;${\it bs}$) $\Leftrightarrow$ agree\_on\_common($T$;${\it as}$;${\it bs}$)))
\\[0ex]\& ($\neg$($x$ $\in$ ${\it as}$) $\Rightarrow$ (agree\_on\_common($T$;${\it as}$;$x$.${\it bs}$) $\Leftrightarrow$ agree\_on\_common($T$;${\it as}$;${\it bs}$)))