public function explain_annuity_certain()
 {
     $escalation_format = new CT1_Interest_Format($this->get_escalation_frequency());
     if ($escalation_format->is_continuous() || $this->get_escalation_frequency() >= $this->get_m()) {
         return $this->explain_annuity_certain_escalation_continual();
     } else {
         return $this->explain_annuity_certain_escalation_stepped();
     }
 }