Suppose you and I are negotiating a contract for the sale of red, white and blue shoe strings.
On Monday, we orally agree that if you buy the shoe strings, I will deliver them in five days from the date of the contract, and that I guarantee that all of the plastic tips on the shoe strings will be blue.
On Tuesday, you and I execute a written agreement under which you will purchase 1,000 red, white and blue shoestrings.
When we meet to sign the written agreement, we also agree that our total agreement will consist of the written agreement and the earlier oral agreement. We do not intend the written agreement to be the complete and exclusive statement of our obligations, so...