Your definition of the atom:source element is looser than that found in 4.2.11 of RFC4287 in that it doesn't specify which elements of a feed's metadata "SHOULD" be preserved and it also doesn't prohibit modification of an atom:source element which exists within a deleted-entry that is being copied. (i.e. in Atom you only "MAY" insert an atom:source if there isn't already one present in the entry.) Do you really intend to loosen the rules for atom:source? If not, then why not just incorporate 4.2.11 by reference in the same way you do for Atom specified elements like atom:link?