Question: Martinson purchased some real estate from the American Realty Company. He paid 25 percent down and signed papers giving American a twenty-year mortgage on the property. After five years, Martinson defaulted (couldn't pay). Martinson's friend Bloomingdale became interested in the property and agreed to assume Martinson's obligation under the terms of the mortgage as it was written, which was acceptable to American Realty. Bloomingdale then paid off the past due mortgage payments plus some other fees, and in turn, American Realty accepted him as the new owner and the person who would continue to make the mortgage payments. American Realty then discharged Martinson from the mortgage contract. What was the legal doctrine used to discharge Martinson from his obligation under the terms of the mortgage?