$\forall$${\it the\_es}$:ES, $l$:IdLnk, ${\it e'}$:E, $m$:(Msg on $l$).
\\[0ex]($m$ $\in$ msgs($l$;before(${\it e'}$))) $\Leftrightarrow$ ($\exists$$e$:E. ($e$ $<$loc ${\it e'}$) \& haslnk($l$;$e$) \& $m$ $=$ emsg($e$))