Enveloping signatures

Hi all,

I'm working on a DSIG implementation and I have
a question which I havn't been able to clear up
definitively:
Which of the following document type declaration is
more appropriate for a DSIG-enveloping-signature-
document?
1. <!DOCTYPE Signature>
2. <!DOCTYPE Signature SYSTEM
"http://www.w3.org/TR/xmldsig-core/xmldsig-core-schema.dtd">

Are both possible?  Should a enveloping
signature always be XML and core validated?
Or is it sufficient to be (only) well-formed and
core validated?

Could anybody help me? Thanks in advance.

Best regards,
/Sven