capture program drop myprobit_lf
program myprobit_lf
version 10.0
args lnf xb
quietly replace `lnf' = ln(normal( `xb' )) ///
if $ML_y1 == 1
quietly replace `lnf' = ln(normal( -`xb' )) ///
if $ML_y1 == 0
end
ml model lf myprobit_lf (foreign weight mpg)
ml check

请问一下，为什么$ML_y1不能被stata替换成foreign？