Question about Enveloping Signatures

In 3.5.1, the requirements doc implies that you can envelope a "particular 
part" of an Input Document.

For example, we could do something like -
  1) client sends list of InputDocuments
  2) client sends list of EnvelopedDocuments:
      2a) each EnvelopedDocument refers to an InputDocument,
      2b) to which it can apply a transform (perhaps just select an element)
  3) client sends list of SignedDocuments:
      3a) each SignedDocument refers to either an InputDocument or 
EnvelopedDocument, and also specifies ds:Transfroms

Processing would be simpler if you could only envelope *entire* 
InputDocuments.  Then we could remove (2b).

QUESTION:  Do we need the capability to envelope particular parts of 
InputDocuments?

The use for enveloping signatures I've seen is mostly enveloping of 
signature properties, which can easily be delivered as individual 
InputDocuments.  Would it be restrictive to limit ourselves to this?


Trevor