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 = ly_scm2rational (scm_cdar (s));
+ Rational old_alter = robust_scm2rational (scm_cdar (s), 0);
if (new_alter_pair == SCM_BOOL_F
|| extranatural
&& (ly_scm2rational (scm_cdr (new_alter_pair)) - old_alter)*old_alter < Rational (0))