(defendants appeal a provision in a contract which was also set out in a deed that created a fee simple determinable; plaintiff, Texas attorney, requested Tenn. attorney to include self-enforcing rights if specific conditions were not fulfilled; plaintiff stated provision was to eliminate the possibility of a lawsuit to protect her rights on the issue; defendants raised defenses of waiver, laches, and impossibility of performance, court rejected; plaintiffs failure to assert right for four years does not constitute a presumed waiver; court noted the literal meaning of "clear" and "unambiguous" language in a contract controls).