The message-supersede command pops up a message buffer that will
supersede the message in the current buffer.
Headers matching the message-ignored-supersedes-headers are
removed before popping up the new message buffer. The default is
‘^Path:\\|^Date\\|^NNTP-Posting-Host:\\|^Xref:\\|^Lines:\\|
^Received:\\|^X-From-Line:\\|^X-Trace:\\|^X-Complaints-To:\\|
Return-Path:\\|^Supersedes:\\|^NNTP-Posting-Date:\\|^X-Trace:\\|
^X-Complaints-To:\\|^Cancel-Lock:\\|^Cancel-Key:\\|^X-Hashcash:\\|
^X-Payment:\\|^Approved:’.