What are the names of patients who are not taking the medication of Procrastin-X.
SELECT name FROM patient EXCEPT SELECT T1.name FROM patient AS T1 JOIN Prescribes AS T2 ON T2.Patient  =  T1.SSN JOIN Medication AS T3 ON T2.Medication  =  T3.Code WHERE T3.name  =  'Procrastin-X'
patients not taking a specific medication refers to using EXCEPT to exclude those who are prescribed that medication; 'Procrastin-X' refers to the name of the medication; joining tables for patient, prescriptions, and medication is necessary to filter out patients based on medication prescribed;