public ParsedSummaryField summaryFieldFor(String name) {
        if (summaryFields.containsKey(name)) {
            return summaryFields.get(name);
        }
        var sf = new ParsedSummaryField(name, getType());
        summaryFields.put(name, sf);
        return sf;
    }
get an existing summary field for modification, or create it
config-model/src/main/java/com/yahoo/schema/parser/ParsedField.java