public Set<Field> fieldSet() {
        return fieldSet(DOCUMENT);
    }
<p>Returns an ordered set snapshot of all fields of this documenttype,
<i>except the fields of Document</i>.
Only the overridden version will be returned for overridden fields.</p>

<p>The fields of a document type has a well-defined order which is
exhibited in this set:
- Fields come in the order defined in the document type definition.
- The fields defined in inherited types come before those in
the document type itself.
- When a field in an inherited type is overridden, the value is overridden,
but not the ordering.
</p>

@return an unmodifiable snapshot of the fields in this type
document/src/main/java/com/yahoo/document/DocumentType.java