$\forall$${\it es}$:ES, $l$:IdLnk, ${\it tg}$:Id, $e$:E. ($\uparrow$($e$ $\in_{b}$ es{-}in{-}port(${\it es}$;$l$;${\it tg}$))) $\Leftarrow\!\Rightarrow$ (kind($e$) = rcv($l$,${\it tg}$) $\in$ Knd)