The only way to do that is to have it be in the car...
Actually, you could have a special symbol '! in the car in order to identify that the cdr is actually supposed to be the last element. Then (a b c (d e f)) is represented as (a . (b . (c . (! . (d . (e . f)))))), i.e. (a b c ! d e f). Banged lists. ;)
In fact, having the symbol be a colon might even make it a nice optional style:
(accum acc
(each x args
(acc (* 2 x))))
(accum acc :
each x args :
acc : * 2 x) ; reminiscent of Arc's (acc:* 2 x)
It's just that you never get to observe the ': from within code. There might be gotchas when generating code, too: The expression (quote : ) would evaluate to nil, huh? XD
