def ARRAY_ELEM_AT(array, idx):
return {'<STR_LIT>': [array, idx]}<EOL>
Returns the element at the specified array index.
See https://docs.mongodb.com/manual/reference/operator/aggregation/arrayElemAt/
for more details
:param array: An expression that can be any valid expression as long as it resolves to an array.
:param idx: An expression can be any valid expression as long as it resolves to an integer.
:return: Aggregation operator