The definition of symbiosis has varied among scientists. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to any type of persistent biological interaction (i.e. mutualistic, commensalistic, or parasitic).
The Golden Pledge, which is a System Plus Warranty with a 20 year leak guarantee, can pay for itself within the first ten years, sometimes many times over. The System Plus transforms the normal warranty by removing the small print.
A pledge is basically very serious formal promise. You can pledge allegiance to your country, you can pledge to keep a secret, and you can pledge a sum of money to a cause. Pledge can be used as both a noun and a verb. As a noun, it can be a solemn promise you've made. Or even the person who makes that promise, like the freshman pledges who take an oath to join a fraternity in college. As a verb, it describes the act of promising.