[Om] OMR xref vs href

[Dan Roozemond]

Dear OpenMath users,

While using OpenMath in the SCIEnce project 
(http://www.symbolic-computation.org) we stumbled upon the following.

The grammar defines OMR's as:
   # reference constructor
   OMR = element OMR { common.attributes,
         attribute href { xsd:anyURI }
   }

but then in examples 'xref' instead of 'href' is used several times, e.g.

<OMOBJ version="2.0">
   <OMA id="bar">
     <OMS cd="arith1" name="plus"/>
     <OMI>1</OMI>
     <OMR xref="baz"/>
   </OMA>
</OMOBJ>

What is the correct use? Is there a difference between xref and href?

One could deduce from the standard that xref could/should/must point to 
a local id, and href could/should/must be a URI, so that
   <OMR xref="baz">
is equivalent to
   <OMR href="#baz">
, but this is nowhere mentioned explicitly.

Kind regards,
Dan Roozemond