function test_is_coupon_used()
 {
     $coupon = new WPSC_Coupon(self::USE_ONCE_COUPON_ID);
     $this->assertTrue($coupon->is_active());
     $this->assertTrue($coupon->is_use_once());
     $this->assertFalse($coupon->is_used());
     $coupon->used();
     $this->assertTrue($coupon->is_used());
     $this->assertFalse($coupon->is_active());
 }