what was the dosage of heparin sodium that was last prescribed to patient 56829 until 11/2105?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 56829 ) and prescriptions.drug = 'heparin sodium' and strftime('%y-%m',prescriptions.startdate) <= '2105-11' order by prescriptions.startdate desc limit 1