2.2. Using namespaces

All namespaces referenced from data objects must be declared in the X-definition header.

<xd:def xmlns:xd = "http://www.xdef.org/xdef/4.2"
        xmlns:bk = "http://www.book.com"
        xd:name = "Books" xd:root = "bk:Inventory" >

  <bk:Inventory>
     <bk:book xd:xcript = "occurs *"
              ISBN      = "num(10);" >
        <bk:publisher> string();  </bk:publisher>
        <bk:year>      gYear();   </bk:year>
        <bk:price>     decimal(); </bk:price>
        <bk:author xd:script="occurs *"> string(); </bk:author>
        <bk:title>     string();  </bk:title>
     </bk:book>    
</bk:Inventory>

</xd:def>

You can try it HERE

Note It is also possible to use the "default" namespace without prefix, but this is strictly not recommended. You should always prefer to declare namespaces with a prefix.