What are their names?
SELECT name FROM patient WHERE SSN NOT IN  (  SELECT T1.patient FROM Prescribes AS T1 JOIN Medication AS T2 ON T1.Medication   =   T2.Code WHERE T2.name   =   'Procrastin-X'  ) 
names of patients not prescribed 'Procrastin-X' medication; using NOT IN for exclusion; joining tables to filter based on medication name;