This also implements RFC-18 and closes http://bugs.musicbrainz.org/ticket/3565

RFC: http://lists.musicbrainz.org/pipermail/musicbrainz-style/2010-February/008692.html

The RFC is a bit complex, and mostly docs-stuff, but the part which needs a Relationship Editor is the modification it makes to a passed AR RFC from 2 years ago which was still waiting for implementation (see the trac ticket). Rather than create a new AR, or ARs, this proposal modified that earlier proposal to make it possible to be implemented without modifications to the AR system.

I could say this in a lot more complex way, but it boils down to this:

For each of the following, add the vocal tree and the instrument tree as "vocal" and "instrument" attributes, like in the performance AR, but overloading the existing AR, not creating 2 new ones. (So "engineered vocal instrument", not "engineered"/"engineered vocal"/"engineered{{instrument}}"):

• Engineer Relationship Type
• Audio Engineer Relationship Type
• Sound Engineer Relationship Type
• Live Sound Engineer Relationship Type
• Mix Engineer Relationship Type
• Recording Engineer Relationship Type
• Mastering Engineer Relationship Type
• Editor Relationship Type

While you're modifying those, the doc link for each Relationship Type above also needs updating. Not all of these have yet been written, but they'll all be done by end of week. (Part of the docs changes I noted above.)
http://wiki.musicbrainz.org/Engineer_Relationship_Type (this one doesn't need changing)
http://wiki.musicbrainz.org/Audio_Engineer_Relationship_Type
http://wiki.musicbrainz.org/Sound_Engineer_Relationship_Type
http://wiki.musicbrainz.org/Live_Sound_Engineer_Relationship_Type
http://wiki.musicbrainz.org/Mix_Engineer_Relationship_Type
http://wiki.musicbrainz.org/Recording_Engineer_Relationship_Type
http://wiki.musicbrainz.org/Mastering_Engineer_Relationship_Type
http://wiki.musicbrainz.org/Editor_Relationship_Type

Please note that these are all both track and release ARs; all 8 of each type need updating.