public Collection<SDField> extraFieldList() {
        if (inherited.isEmpty()) return fields.values();
        var fields = new HashSet<>(requireInherited().extraFieldList());
        fields.addAll(this.fields.values());
        return fields;
    }
Adds an extra field of this search definition not contained in a document

@param field to add to the schemas list of external fields
config-model/src/main/java/com/yahoo/schema/Schema.java