f{-}rel\{\$z,\$wanted\}(${\it es}$;$L$;$e_{1}$;$e_{2}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$($\neg$(loc($e_{2}$) = loc($e_{1}$))) c$\wedge$ the rcv(wanted message from $e_{1}$ to loc($e_{2}$)) $\leq$loc $e_{2}$