SCM new_alter_pair = scm_assoc (scm_caar (s), key);
Rational old_alter = robust_scm2rational (scm_cdar (s), 0);
if (new_alter_pair == SCM_BOOL_F
SCM new_alter_pair = scm_assoc (scm_caar (s), key);
Rational old_alter = robust_scm2rational (scm_cdar (s), 0);
if (new_alter_pair == SCM_BOOL_F