$\forall$$L$:MsgA List, $l$:IdLnk, ${\it tg}$:Id.
\\[0ex]($\forall$$A$,$B$$\in$$L$.$A$ $\Vert\!+$ $B$) $\Rightarrow$ (rcv($l$,${\it tg}$) declared in $\oplus$($L$) $\Leftrightarrow$ ($\exists$$M$$\in$$L$.rcv($l$,${\it tg}$) declared in $M$))