[Old-standards] Remove from list: RFC1847 Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted

[Harald Tveit Alvestrand]

RFC1847       Security Multiparts for MIME: Multipart/Signed and 
Multipart/Encrypted

multipart/signed is referenced by the S/MIME and OpenPGP specifications. 
Multipart/encrypted is AFAIK unused, but we're not doing half documents.