what was the first value of the anion gap lab test of patient 30525 during the last hospital visit.
select labevents.valuenum from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 30525 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'anion gap' ) order by labevents.charttime asc limit 1