McMullen Contractors made a contract with Richardson to build an apartment house for a specific price. A number of serious apartment house fires broke out in the city, and the city council adopted an ordinance increasing the fire precautions that had to be taken in the construction of a new building. Compliance with these new requirements would make the construction of the apartment house for Richardson more expensive than McMullen had originally contemplated. Is McMullen discharged from the contract to build the apartment house?