public function get_merge_codes() {

		if ( ! isset( $this->merge_codes ) ) {
			$codes = array_merge( $this->get_merge_code_defaults(), $this->set_merge_codes() );
			asort( $codes );
			$this->merge_codes = $codes;
		}

		return apply_filters( $this->get_filter( 'get_merge_codes' ), $this->merge_codes, $this );

	}