@Override
        protected boolean shouldConvert(Expression exp) {
            if ( ! (exp instanceof OutputExpression)) {
                return false;
            }
            String fieldName = ((OutputExpression)exp).getFieldName();
            if (fieldName == null) {
                return true; // inject appropriate field name
            }
            if ( validate && ! fieldName.equals(field.getName())) {
                fail(schema, field, "Indexing expression '" + exp + "' attempts to write to a field other than '" +
                                    field.getName() + "'.");
            }
            return false;
        }
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