private List<OperatorNode<ExpressionOperator>> getReadFieldExpressions(OperatorNode<ExpressionOperator> in) {
        List<OperatorNode<ExpressionOperator>> readFieldList = new ArrayList<>();
        switch (in.getOperator()) {
            case READ_FIELD -> readFieldList.add(in);
            case CALL -> {
                List<OperatorNode<ExpressionOperator>> callArgs = in.getArgument(1);
                for (OperatorNode<ExpressionOperator> callArg : callArgs) {
                    if (callArg.getOperator() == ExpressionOperator.READ_FIELD) {
                        readFieldList.add(callArg);
                    }
                }
            }
        }
        return readFieldList;
    }
Scans the given node for READ_FIELD expressions.

TODO: Search recursively and consider additional operators

@param in the node to scan
@return list of READ_FIELD expressions
container-search/src/main/java/com/yahoo/search/yql/ProgramParser.java