program define mylogit1_lf
	version 8.0 
	args lnf xb
	quietly replace `lnf' =  - $ML_y1 * ln(1+exp(-`xb')) if $ML_y1==1
	quietly replace `lnf' = - (1 - $ML_y1)* ln(1 + exp(`xb')) if $ML_y1==0
end