Value ExpressionFilter::serialize(bool explain) const {
    return Value(DOC("$filter" << DOC("input" << _input->serialize(explain) << "as" << _varName
                                              << "cond" << _filter->serialize(explain))));
}
Value ExpressionFilter::serialize(bool explain) const {
    return Value(DOC("$filter" << DOC("input" << _input->serialize(explain) << "as" << _varName
                                              << "cond" << _filter->serialize(explain))));
}