st: maximum likelihood estimation unknown function error

["Marcelo Cadena" <mcadena@uni-bonn.de>]

I am trying to maximize the following likelihood function 
implemented in my do-file below. In my opinion the 
structure of the program corresponds to the one given in 
the book of  Gould and Sribney, "Maximum likelihood 
estimation with Stata". However, when checking the code, 
stata returns the error message given at the bottom.
The variables in sample1 are named ML_y1 to ML_y5. I would 
be grateful if somebody can find the error in my code




. program define itrader
  1.         version 6
  2.         args lnf theta1 theta2 theta3 theta4
  3.
.         tempvar w q v
  4.         quietly gen double `w' = 
exp(ln(1-`theta1')-2*`theta3'*$ML_y3  + ($ML_y1 + 
$ML_y2)*ln(`theta3'*$ML_y3) -

>  $ML_y4 - $ML_y5)
  5.         quietly gen double `q' = 
exp(ln(`theta1'*`theta2') - 2*`theta3'*$ML_y3 - 
`theta4'*$ML_y3 + $ML_y1*ln(`th

> eta3'*$ML_y3) + $ML_y2*ln((`theta4' + `theta3')*$ML_y3)- $ML_y4 - $ML_y5)
  6.         quietly gen double `v' = 
exp(ln(`theta1'*(1-`theta2')) - 2*`theta4'*$ML_y3 - 
`theta3'*$ML_y3 + $ML_y1*ln

> ((`theta4' + `theta3')*$ML_y3) + $ML_y2*ln(`theta3'*$ML_y3)- $ML_y4 - $ML_y5)
  7.
.         quietly replace `lnf' = `w' + `q' + `v'
  8. end

. 
. use 
"C:\DOKUME~1\JUANMA~1\EIGENE~1\prog\MATHEP~1\Stata8se\sample1.dta"

. ml model lf itrader () () () ()

. 
end of do-file

. ml check

Test 1:  Calling itrader to check if it computes log 
likelihood and
         does not alter coefficient vector...
         FAILED; itrader returned error 133.

Here is a trace of its execution:
------------------------------------------------------------------------------
-> itrader __000009 __000005 __000006 __000007 __000008
            - `begin'
            = capture noisily version 8: itrader __000009 
__000005 __000006 __000007 __000008
              ----------------------------------------------------------------------------------- 
begin itrader ---
              - version 6
              - args lnf theta1 theta2 theta3 theta4
              - tempvar w q v
              - quietly gen double `w' = 
exp(ln(1-`theta1')-2*`theta3'*$ML_y3 + ($ML_y1 + 
$ML_y2)*ln(`theta3'*$ML_y

> 3) - $ML_y4 - $ML_y5)
              = quietly gen double __00000A = 
exp(ln(1-__000005)-2*__000007* + ( + )*ln(__000007*) -  - 
)

Unknown function +()

              ------------------------------------------------------------------------------------- 
end itrader ---
            - `end'
            = set trace off
------------------------------------------------------------------------------
Fix itrader.
r(133);
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