Saginaw Valley State University has available housing for its students which SVSU offers by contract. Under the contract, before the SVSU is obligated to assign you to a townhouse, the renter must be a full time student of the University. When Rachel signs a contract with SVSU to rent a townhouse, she is only a part-time student. Despite a signed contract, SVSU refuses to assign her to a townhouse. What legal concept describes the 'full time student' obligation which authorizes SVSU to refuse to provide housing to Rachel