51. Mastrangelo and Johnson entered into a written contract for the purchase of an apartment building built by Johnson. The contract was carefully drafted to set forth the agreement of the parties. It was signed by Johnson. Johnson subsequently claimed that the contract was not valid because it was not signed by Mastrangelo and also did not cover all the terms included in the written and oral agreements that the parties had made during their prior negotiations. Mastrangelo claimed that Johnson was barred from introducing proof of all of their prior agreements. Discuss all issues