How many of the patients who underwent a 'bone immobilization' procedure have a driver's license?
patients who underwent a 'bone immobilization' procedure refer to PATIENT from procedures where DESCRIPTION = 'Bone immobilization'; have a driver's license refers to drivers is not null;
SELECT COUNT(DISTINCT T1.patient) FROM patients AS T1 INNER JOIN procedures AS T2 ON T1.patient = T2.PATIENT WHERE T2.DESCRIPTION = 'Bone immobilization' AND T1.drivers IS NOT NULL