@Override
                protected void doVisit(Expression exp) {
                    if (!(exp instanceof AttributeExpression)) {
                        return;
                    }
                    String fieldName = ((AttributeExpression)exp).getFieldName();
                    if (fieldName == null) {
                        fieldName = getName();
                    }
                    Attribute attribute = attributes.get(fieldName);
                    if (attribute == null) {
                        addAttribute(new Attribute(schemaName, fieldName, fieldName, getDataType()));
                    }
                }
Sets the indexing script of this, or null to not use a script
config-model/src/main/java/com/yahoo/schema/document/SDField.java