What is is the number of patients who are not using Procrastin-X?
SELECT count ( * )  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'  ) 
number of patients not using Procrastin-X refers to COUNT(*) WHERE patient's SSN NOT IN the subset of patients prescribed Procrastin-X;