@Override
    public void visitMembers(ObjectVisitor visitor) {
        super.visitMembers(visitor);
        visitor.visit("headertype", contentStructType);
        visitor.visit("inherits", inherits);
    }
Returns an iterator over all fields in this documenttype

@return An iterator for iterating the fields in this documenttype.
document/src/main/java/com/yahoo/document/DocumentType.java