Somebody else could explain it better than I, but there are some hard to describe interactions with evaluation brackets and concatenation. However, in practice, the formula that gets used the most looks like this:
a [ $+ [ b ] $+ [ c ] ]
set -e %foo. $+ $cid $+ $chan Hello World.
echo -a %foo. [ $+ [ $cid ] $+ [ $chan ] ]
set -e $+(%,foo.,$cid,$chan) Hello World.
echo -a $($+(%,foo.,$cid,$chan),2)