Here is a proposal for converting between representation of pairs and
lists.  -- David

(pair->pair pair)      procedure

Returns a pair whose car and cdr fields are the same (in the sense of
eqv?) as the given pair.

In systems in which random-access pairs are disjoint from sequential
pairs, this procedure converts between representations, i.e. given a
sequential pair, it produces a random-access pair.  Given a
random-access pair, it produces a sequential pair.

In systems in which all pairs are random-access pairs, this procedure
behaves as an identity function on pairs.

This operation must take O(1) time.  The resulting pair may be newly
allocated.

(list->list list)      procedure

Returns a list whose elements are the same (in the sense of eqv?) as the
elements of the given list.

In systems in which random-access pairs are disjoint from sequential
pairs, this procedure converts between representations, i.e. given a
sequential list, it produces a random-access list.  Given a
random-access list, it produces a sequential list.

In systems in which all pairs are random-access pairs, this procedure
behaves as an identity function on lists.

This operation may take O(n) time, where n is the size of the list.  The
resulting list may be newly allocated.