how much insulin had been prescribed to patient 3929 in total this month?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 3929 ) and prescriptions.drug = 'insulin' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month')