Strictly speaking, you don't have to make it a base type
because some systems don't process XSD, so an attribute's
constraints in XML, per se, are the constraints of the
application in the case of a well-formed instance or
of a DTD given a valid instance under strict XML
interpretation.
However, given XSD, that is a good way to do it
and possibly useful. XSD is somewhat controversial
but I've no objection to its use here.
len
From: Winfield Wagner [mailto:]
If the reference system is considered a identified form of measurement or a
distinct qualifier it should be an attribute. It is metadata describing a
specific quality or context of the element. In order to do this you must
make a reference system element its own separate base type. It can not
belong in the realm of type text or type code. The reason is metadata
attributes should be connected to the type and not just to an element name
(reuse). It is not good practice to have single elements with their on
special metadata attributes.
