def excel_key(index):
X = lambda n: ~n and X((n // <NUM_LIT>)-<NUM_LIT:1>) + chr(<NUM_LIT> + (n % <NUM_LIT>)) or '<STR_LIT>'<EOL>return X(int(index))<EOL>
create a key for index by converting index into a base-26 number, using A-Z as the characters.