why doesn't this work
//set %abc 1 | var %a = abc | unset -s $+(%,%a)

while this does:
//set %abc 1 | var %a = abc | unset -s $+(%,[ %a ])

we concluded mirc prolly sort of "disables" evaluating %vars. But imo there's no need to do that when the vars are inside an identifier.

Pretty much the same problem as this thread.