(encycle! list head-size cycle-size)
Head-size and cycle-size MUST be exact non-negative integers. The (possibly improper) list MUST contain at least head-size + cycle-size pairs. If cycle-size = 0, does nothing. If cycle-size > 0, mutates list to have a cyclic prefix of length head-size and cycle length cycle-size, by setting the cdr of the (head-size + cycle-size)th pair in the list to refer to the (head-size+ 1)th pair in the list. Returns unspecified values.
What else should go here depends on what WG1 decides to do about circular lists generally.