what is the amount of bisacodyl dose prescribed to patient 16554 since 01/2105?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 16554 ) and prescriptions.drug = 'bisacodyl' and strftime('%y-%m',prescriptions.startdate) >= '2105-01'