how many patients were diagnosed with fracture one rib-closed and did not come back to hospital within 2 months until 4 years ago?
select ( select count( distinct t1.subject_id ) from ( select admissions.subject_id, diagnoses_icd.charttime from diagnoses_icd join admissions on diagnoses_icd.hadm_id = admissions.hadm_id where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'fracture one rib-closed' ) and datetime(diagnoses_icd.charttime) <= datetime(current_time,'-4 year') ) as t1 ) - ( select count( distinct t2.subject_id ) from ( select admissions.subject_id, diagnoses_icd.charttime from diagnoses_icd join admissions on diagnoses_icd.hadm_id = admissions.hadm_id where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'fracture one rib-closed' ) and datetime(diagnoses_icd.charttime) <= datetime(current_time,'-4 year') ) as t2 join admissions on t2.subject_id = admissions.subject_id where t2.charttime < admissions.admittime and datetime(admissions.admittime) <= datetime(current_time,'-4 year') and datetime(admissions.admittime) between datetime(t2.charttime) and datetime(t2.charttime,'+2 month') )