def SUM(*expression):
return {'<STR_LIT>': list(expression)} if len(expression) > <NUM_LIT:1> else {'<STR_LIT>': expression[<NUM_LIT:0>]} if expression else {}<EOL>
Calculates and returns the sum of numeric values.
See https://docs.mongodb.com/manual/reference/operator/aggregation/sum/
for more details
:param expression: expression or variables
:return: Aggregation operator