Prefixes must be declared by associated prefixes with namespaces "before" they are used.

You can only declare this association on elements.

The syntax is: xmlns:prefix="some:uri"

Example:

<c:pseudocode xmlns:c="urn:publicid:IDN+mathdoc.org:pseudocode:en">

   <c:comment xlink:href="http://somewhere..." 
              xmlns:xlink="http://www.w3.org/..."/>

</c:pseudocode>

"before" means on the element where the prefix occurs or on an ancestor element.

The 'xml' prefix is pre-defined to be http://www.w3.org/XML/1998/namespace