what's the first value of the base excess lab test of patient 15968 until 11/2103?
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 15968 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'base excess' ) and strftime('%y-%m',labevents.charttime) <= '2103-11' order by labevents.charttime asc limit 1