is mentioned by

Thm* the_w:World, l:IdLnk, i:Id, a:Action(i). Thm* isrcv(l;a)  isnull(a) & isrcv(kind(a)) & lnk(kind(a)) = l	[assert-w-isrcvl]
Def kind(e) == kind(act(e))	[w-ekind]
Def msg(a) == msg(lnk(kind(a));tag(kind(a));val(a))	[w-msg]
Def isrcv(l;a) == isnull(a)isrcv(kind(a))lnk(kind(a)) = l	[w-isrcvl]
Def valtype(i;a) == kindcase(kind(a);a.w.TA(i,a);l,tg.w.M(l,tg))	[w-valtype]

Try larger context: EventSystems IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html