msg{-}spec{-}loc(${\it snd}$;$i$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$l$:IdLnk. ($l$ $\in$ msg{-}spec{-}links(${\it snd}$) $\in$ IdLnk) $\Rightarrow$ source($l$) $=$ $i$ $\in$ Id