for (SCM s = last; scm_is_pair (s); s = scm_cdr (s))
{
SCM new_alter_pair = scm_assoc (scm_caar (s), key);
Rational old_alter = robust_scm2rational (scm_cdar (s), 0);
for (SCM s = last; scm_is_pair (s); s = scm_cdr (s))
{
SCM new_alter_pair = scm_assoc (scm_caar (s), key);
Rational old_alter = robust_scm2rational (scm_cdar (s), 0);