Support for the <result> inside <procedure> elements

Hi there,
I’m currently using Docbook 5.1b7 and have found that I really want to
be able to use the <result> tag, as described in the 5.1 guide here:

http://docbook.org/tdg51/en/html/result.html 

The DocBook guide says that the <result> tag is supported but it doesn’t
seem to be supported by the docbookxi.rng schema. 

Putting a <result> tag inside a <procedure> generates this error when
validating:

    element "result" not allowed anywhere; expected the element end-tag
    or element "step"

Would anyone be able to advise me how to modify the schema to enable the
use of the <result> tag?

Thanks for your help,

Graeme