$\forall$$M_{1}$, $M_{2}$:MsgA, $l$:IdLnk.
\\[0ex]($\uparrow$$M_{1}$ $\oplus$ $M_{2}$ sends on link $l$) $\Leftarrow\!\Rightarrow$ (($\uparrow$$M_{1}$ sends on link $l$) $\vee$ ($\uparrow$$M_{2}$ sends on link $l$))