ds:Reference @URI attribute

Hello,

In a ds:Signature, enveloped in a dss:SignatureObject in a dss:SignResponse,
can or should the URI attribute be present?  If yes, what should be its
value?  

According to http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-URI, I
would assume the attribute could be omitted as "the receiving application is
expected to know the identity of the object", via the link between the
SignResponse document to its corresponding SignRequest document.  Is that
correct?

If there are multiple dss:Document elements in a dss:InputDocuments
structure, how does one correlate a ds:Signature element in a
dss:SignResponse to the corresponding dss:Document in the dss:SignRequest?

My first idea was to use the value of the ID attribute on the dss:Document
in dss:InputDocuments. However, ID is an XML ID type attribute, and the URI
attribute has type anyURI. The ID attribute will not be in the
dss:SignResponse when the document to be signed is included in the
SignRequest but not in the SignResponse (which happens when not using the
DocumentWithSignature output element). 

Pim