@Override
        protected Expression doConvert(Expression exp) {
            List<Expression> ret = new LinkedList<>();
            if (exp instanceof AttributeExpression) {
                ret.add(new AttributeExpression(field.getName()));
            } else if (exp instanceof IndexExpression) {
                ret.add(new IndexExpression(field.getName()));
            } else if (exp instanceof SummaryExpression) {
                ret.add(new SummaryExpression(field.getName()));
            } else {
                throw new UnsupportedOperationException(exp.getClass().getName());
            }
            return new StatementExpression(ret);
        }
This processor modifies all indexing scripts so that they output to the owning field by default. It also prevents
any output expression from writing to any field except for the owning field. Finally, for <code>SummaryExpression</code>,
this processor expands to write all appropriate summary fields.

@author Simon Thoresen Hult
config-model/src/main/java/com/yahoo/schema/processing/IndexingOutputs.java