how much potassium chl 40 meq / 1000 ml d5 1/2 ns was prescribed to patient 73713, in total in this hospital visit?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 73713 and admissions.dischtime is null ) and prescriptions.drug = 'potassium chl 40 meq / 1000 ml d5 1/2 ns'