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)