what was the total dose of furosemide patient 72909 had been prescribed during 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 = 72909 ) and prescriptions.drug = 'furosemide' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-0 month')