def FILTER(_input, _as, cond):
    """
    Selects a subset of an array to return based on the specified condition.     Returns an array with only those elements that match the condition.
    The returned elements are in the original order.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/filter/
    for more details
    :param _input: An expression that resolves to an array.
    :param _as: A name for the variable that represents each individual element of the input array.
    :param cond: An expression that resolves to a boolean value used to determine if an element should be included in the output array.
    :return: Aggregation operator
    """
    return {'$filter': {'input': _input, 'as': _as, 'cond': cond}}
