"Supersedes:" is not documented in any RFC that I am aware, however it
is in widespread use in netnews. It is documented in Henry Spencer's
draft follow-on document to 1036.
The header takes as a value a single message-id, which the message is
to replace. I believe the follow-on document may allow multiple
whitespace-separated message-ids, but existing software doesn't handle
this.
--
_.John G. Myers Internet: jgm+(_at_)CMU(_dot_)EDU
LoseNet: ...!seismo!ihnp4!wiscvm.wisc.edu!give!up