protected void derive(SDDocumentType document, Schema schema) {
        for (Field field : document.fieldSet()) {
            SDField sdField = (SDField) field;
            if ( ! sdField.isExtraField()) {
                derive(sdField, schema);
            }
        }
    }
Derives the content of this configuration. This
default calls derive(SDField) for each document field
config-model/src/main/java/com/yahoo/schema/derived/Derived.java