How do I loop excel sheet names inside quotes in GAMS 'GDXXRW?

Question

I would like to define a string set in GAMS to loop to read different xlsx sheet names in gdxxrw. However, I didn't find the appropriate code in the help documentation using string formatting and String Substitution as keywords.

'''

SET  
sheets /index1, index2, index3/ 
B /a, b, c/ 
; 

PARAMETER 
POP0(B) 
POP(B) 
; 
LOOP(sheets, 
* Write Excel data to GDX
put_utility 'exec' / 'gdxxrw.exe Input.xlsx se=0 index='sheets.tl'!a1';  
$Load POP0 
POP(B)=POP0(B); 
execute_unload "Output.gdx"  
POP 
* Write GDX data to excel
  put_utility 'exec' / 'gdxxrw.exe RESULT.gdx o=RESULT.xlsx index='sheets.tl'!a7';

''' I read loop in GAMS for scenario generation in excel. But GAMS report error as that '''

116  Label is unknown
141  Symbol declared but no values have been assigned. Check for missing
        data definition, assignment, data loading or implicit assignment
        via a solve statement.
        A wild shot: You may have spurious commas in the explanatory
        text of a declaration. Check symbol reference list.
300  Remaining errors not printed for this line
352  Set has not been initialized
353  The domain for this index position is unknown and the element
        cannot be checked at this point. Missing data statement.
502  GDXIN  file not open - ignore rest of line
509  Dollar control statements are processed during compilation
        and NOT during execution. The use of dollar statements
        that change data may be misleading when inside a LOOP
        or IF statement.

''' I think the key is the apply of $Load or $loaddc. Does GAMS have a string formatting method control index sheet in quotes?

Answer 1

After a night of debugging, I finally solved the problem. '''

SET  
sheets /index1, index2, index3/ 
B /a, b, c/ 
; 

PARAMETER 
POP0(B) 
POP(B) 
; 
LOOP(sheets, 
* Write Excel data to GDX
put_utility 'exec' / 'gdxxrw.exe Input.xlsx se=0 index='sheets.tl:0'!a1';
*":0" is needed
* Load data from GDX (substitution of $load)
  execute_load 'data.gdx', 
POP0
;  

POP(B)=POP0(B); 
* Load data from GDX 
execute_unload "Output.gdx", 
POP
;  
* Write GDX data to excel
put_utility 'exec' / 'gdxxrw.exe RESULT.gdx o=RESULT.xlsx index='sheets.tl:0'!a7';

'''

Very thank Lutz!