@Override
    public List<SDField> allConcreteFields() {
        List<SDField> allFields = new ArrayList<>();
        allFields.addAll(extraFieldList());
        for (Field field : documentType.fieldSet()) {
            allFields.add((SDField)field);
        }
        return allFields;
    }
Returns a list of all the fields of this schema, that is all fields in all documents, in the documents
they inherit, and all extra fields. The caller receives ownership to the list - subsequent changes to it will not
impact this.
config-model/src/main/java/com/yahoo/schema/Schema.java