what is the first time patient 83901 received a prescription for lactulose enema in the previous month?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 83901 ) and prescriptions.drug = 'lactulose enema' and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-1 month') order by prescriptions.startdate asc limit 1