Consider preserving the tabification in your diff -- that would make
the substantive change easier to see for reviewers.  Only one variable
rename is substantive to avoid shadowing, list-b ---> pair-b; the
others, = ---> elt= and list-a ---> pair-a are not, though the latter
is justifiable to make in the same patch.  Perhaps the word `tail' is
more appropriate than `pair' here, since tail-a is a tail of (and
possibly identical to) list-a and likewise tail-b/list-b.