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(); } }