（1）
use chem_patents_maindataset.dta, clear
forvalues x=1876/1939 {
    gen td_`x'=0
    qui replace td_`x'=1 if grn==`x'
    }

sort uspto_class grn
bys uspto: gen ccc=sum(count)
foreach var in count_usa count  {
    qui replace `var'=. if ccc==0
    }
sum count_usa
order count ccc count_usa



（2）
use chem_patents_maindataset.dta, clear
forvalues x=1876/1939 {
    gen td_`x'=0
    qui replace td_`x'=1 if grn==`x'
    }

sort uspto_class grn
bys uspto: gen ccc=sum(count)
foreach var in count_usa   {
    qui replace `var'=. if ccc==0
    }
sum count_usa

order count ccc count_usa




（3）
use chem_patents_maindataset.dta, clear
bys uspto: gen ccc=sum(count)
replace count_usa = . if ccc == 0
sum count_usa
order count ccc count_usa