def LET(_in, _vars):
return {'<STR_LIT>': {'<STR_LIT>': _vars, '<STR_LIT>': _in}}<EOL>
Binds variables for use in the specified expression, and returns the result of the expression.
See https://docs.mongodb.com/manual/reference/operator/aggregation/let/
for more details
:param _in: The expression to evaluate.
:param _vars: Assignment block for the variables accessible in the in expression.
:return: Aggregation operator