forvalues命令问题

我想用stata编辑一个循环命令来筛选出最好的控制变量，但是命令一直报错：unknown function combin()
invalid syntax

local varlist dominst domper Ln_Ksize SOE Liquidity Ln_RE FirmAge FIXED Share Market Growth
local k=7
local max_r2=0
local best_vars

forvalues i=1/`=combin(`=wordcount("`$varlist'")',`k')'{
     local vars:word `i'of `=varlist'
     reghdfe depvar ` Lntotal ',a(year ind1) vce(r)
     if e(r2)>`max_r2'{
         local max_r2=e(r2)
         local best_vars="`vars'"
    }
}

display "best variable combination:" `best_vars'
display "Max R2 value:" `max_r2'