asking for help urgently: stata ml programming

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Dear statalist, I 've been trying to use 'stata ml' command to maximize the following equation

(k1i + e1i)X1i + (k2i + e2i)X2i + uX1iX2i

if u=0 then the equation can be estimated by standard 'bivariate probit'. However, when u>0 the decision of (X1i, X2i) = (1,1) or (0,0) will be biased; when u>0 the decision of (X1i, X2i) = (1,0) or (0,1) will suffer the biaseness also. 

My first question is of how and where to put the restriction of u > 0 and u< 0 when i am programming ml.

Second question is that do i have to write theta3 for uX1iX2i? if i do, how should i put it?  any suggestion or help will be very much appreciated. thank you!

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关键词：Programming urgently Program urgent R help Programming Stata Help Asking urgently

is it correct to express in this way?

* theta1=X1Beta1

* theta2=X2Beta2

* theta4=rho12

* theta3=X1X2Beta3

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qui replace `lnf'=ln(binorm(`theta1'+`theta3',`theta2'+`theta3',`rho12')) if $ML_y1==1 & $ML_y2==1 & `theta3'<=0

qui replace `lnf'=ln(binorm(`theta1'+`theta3',`theta2'+`theta3',`rho12')-`biv1') if $ML_y1==1 & $ML_y2==1 & `theta3'>0

qui replace `lnf'=ln(binorm(-`theta1',-`theta2',`rho12')) if $ML_y1==0 & $ML_y2==0 & `theta3'<=0

qui replace `lnf'=ln(binorm(-`theta1',-`theta2',`rho12')+`biv2') if $ML_y1==0 & $ML_y2==0 & `theta3'>0

qui replace `lnf'=ln(binorm(`theta1',-`theta2'-`theta3',-`rho12')) if $ML_y1==1 & $ML_y2==0 & `theta3'>=0

qui replace `lnf'=ln(binorm(`theta1',-`theta2'-`theta3',-`rho12')-`biv3') if $ML_y1==1 & $ML_y2==0 & `theta3'<0

qui replace `lnf'=ln(binorm(-`theta1'-`theta3',`theta2',-`rho12')) if $ML_y1==0 & $ML_y2==1 & `theta3'>=0

qui replace `lnf'=ln(binorm(-`theta1'-`theta3',`theta2',-`rho12')-`biv4') if $ML_y1==0 & $ML_y2==1 & `theta3'<0