*设x1、x2、x3无缺失值
foreach v of var x1 x2 x3{
egen min`v'=min(`v')
egen max`v'=max(`v')
}
drop if x1==minx1|x1==maxx1|x2==minx2|x2==maxx2|x3==minx3|x3==maxx3
drop min* max*

*或
foreach v of var x1 x2 x3{
sort `v'
g m`v'=(`v'==`v'[1]|`v'==`v'[_N])
}
keep if mx1+mx2+mx3==0
drop mx1 mx2 mx3

foreach v of var x1 x2 x3 {
sum `v'
gen `v'_1=(`v'==r(max) | `v'==r(min))
}
egen id=rowtotal(x1_1 x2_1 x3_1)
drop if id!=0
drop x?_? id