@Override
    public DocumentFieldNode clone() {
        DocumentFieldNode obj = (DocumentFieldNode)super.clone();
        if (result != null) {
            obj.result = (ResultNode)result.clone();
        }
        return obj;
    }
Sets the name of the field whose value to retrieve.

@param fieldName The field name to set.
searchlib/src/main/java/com/yahoo/searchlib/expression/DocumentFieldNode.java