以下一段stata程序执行到最后提示出错，请告知如何改正，谢谢。
capture program drop hw2question2
program hw2question2
args  lnf a1 a2 a3 a4 a5 a6 a7 g1 g2 g3 g4 g5 g6 g7
tempvar phi1 phi2 phi3 phi4 phi5 phi6 phi7
quietly {
         generate double `phi1' = `a1' * $ML_y2 - `g1'
   generate double `phi2' = (`a1' - `a2') * $ML_y2 - `g1' - `g2'
   generate double `phi3' = (`a1' - `a2' - `a3') * $ML_y2 - `g1' - `g2' - `g3'
   generate double `phi4' = (`a1' - `a2' - `a3' - `a4') * $ML_y2 - `g1' - `g2' - `g3' - `g4'
   generate double `phi5' = (`a1' - `a2' - `a3' - `a4' - `a5') * $ML_y2 - `g1' - `g2' - `g3' - `g4' - `g5'
   generate double `phi6' = (`a1' - `a2' - `a3' - `a4' - `a5'- `a6') * $ML_y2 - `g1' - `g2' - `g3' - `g4' - `g5' - `g6'
   generate double `phi7' = (`a1' - `a2' - `a3' - `a4' - `a5'- `a6' - `a7') * $ML_y2 - `g1' - `g2' - `g3' - `g4' - `g5' - `g6' - `g7'
   replace `lnf' = ln(1 - normal(`phi1')) if $ML_y1 == 0
         replace `lnf' = ln(normal(`phi1') - normal(`phi2')) if $ML_y1 == 1
         replace `lnf' = ln(normal(`phi2') - normal(`phi3')) if $ML_y1 == 2
         replace `lnf' = ln(normal(`phi3') - normal(`phi4')) if $ML_y1 == 3
         replace `lnf' = ln(normal(`phi4') - normal(`phi5')) if $ML_y1 == 4
         replace `lnf' = ln(normal(`phi5') - normal(`phi6')) if $ML_y1 == 5
         replace `lnf' = ln(normal(`phi6') - normal(`phi7')) if $ML_y1 == 6
         replace `lnf' = ln(normal(`phi7')) if $ML_y1 == 7
         }
end
ml model if hw2question2 (nfirm population = ) (a2: ) (a3: ) (a4: ) (a5: ) (a6: ) (a7: ) (g1: ) (g2: ) (g3: ) (g4: ) (g5: ) (g6: ) (g7: )
ml maximize
ml display
program drop hw2question2

执行结果：

. ml model if hw2question2 (nfirm population = ) (a2: ) (a3: ) (a4: ) (a5: ) (a6: ) (a7: ) (g1: ) (g2: ) (g3: ) (g4: ) (g5: ) (g6: ) (g7: )
model required
r(100);
end of do-file