public SDField getExtraField(String fieldName) {
        SDField field = fields.get(fieldName);
        if (field != null) return field;
        if (inherited.isEmpty()) return null;
        return requireInherited().getExtraField(fieldName);
    }
Returns a field by name, or null if it is not present

@param fieldName the name of the external field to get
@return the SDField of this name
config-model/src/main/java/com/yahoo/schema/Schema.java