pred!($e$;${\it e'}$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$(($\neg$($\uparrow$first(${\it e'}$))) c$\wedge$ ($e$ = pred(${\it e'}$))) $\vee$ (($\uparrow$rcv?(${\it e'}$)) c$\wedge$ ($e$ = sender(${\it e'}$)))