def COND(_if, _then, _else):
return {'<STR_LIT>': {'<STR_LIT>': _if, '<STR_LIT>': _then, '<STR_LIT>': _else}}<EOL>
Evaluates a boolean expression to return one of the two specified return expressions.
See https://docs.mongodb.com/manual/reference/operator/aggregation/cond/
for more details
:param _if: expression that will be evaluated
:param _then: if _if expression evaluates as true then COND operator returns _then statement
:param _else: otherwise returns _else statement
:return: Aggregation operator