Some of my files have a REMIXER tag. If one person remixed the song, it will contain that person's name. If, say, both Albert Deal and Torben Schmidt remixed the song, it will contain their comma-separated names, "Albert Deal, Torben Schmidt".

What I'd like to accomplish is a remixer view on my album list which lists tracks organized by remixer. This is pretty easy: %remixer%|%title% basically. However, I'd really like to separate out the lists. For example, Alexander Odden remixed two songs, and he collaborated with STP remixing one song (so my tag reads "Alexander Odden, STP".) I'd like to have an Alexander Odden branch with all three songs, and an STP branch with that one song. I don't see a clear way to do this given the provided descriptions of branching functions, but it seems like a reasonable thing to desire, so I wanted to check and see: Is this possible?

You can use these strings to extract first and last Remixer:
$substr(%remixer%,1,$add($strchr(%remixer%,','),-1))
$trim($substr(%remixer%,$add($strchr(%remixer%,','),1),$len(%remixer%)))

But I have no idea how to let foobar display first and last remixer below the other in one level...
I think you'll need two AlbumList settings: one for first and one for last
Maybe someone other has a idea...

Split the remixer field into multiple values, and use %<remixer>% in your album list preset.

Great; I went through and split up the remixer field (I should have done it that way in the first place, anyway) and now that works fine. The only problem is that my album list tree now has a "?" branch containing all the tracks that don't have a remixer tag (the majority of tracks, really.) My first inclination was to follow the wiki's directions and use the $meta_branch_remap() function which is described as "This is identical to %<name>% except it does not produce a "?", if the tag does not exist."

Well, that sounds like just what I want, but when I use $meta_branch_remap(remixer)|%title% it still produces a branch full of no-remixer-tag files; it just has a blank name, instead of the name "?". Why doesn't that branch get left out of the tree, as the wiki implies it should? ("If only empty values exist, the track will be excluded from the generated album list tree.")

You are right, an absent field and one with an empty value are treated inconsistently. We will consider changing that in the future.

Oh, I understand, so if I had an empty REMIXER tag on the rest of my files they wouldn't show up on the list. That makes some sense.