void addSummaryField(ParsedSummaryField summaryField) {
        String fieldName = summaryField.name();
        verifyThat(! summaryFields.containsKey(fieldName), "already has summary field", fieldName);
        if (summaryField.getType() == null) {
            summaryField.setType(getType());
        }
        summaryFields.put(fieldName, summaryField);
    }
get an existing summary field for modification, or create it
config-model/src/main/java/com/yahoo/schema/parser/ParsedField.java