Dynamic completion question

[Jesse W <jessw@xxxxxxxxxxx>]

I'm working on adding the ability to handle FLAC tags dynamically with 
the completion I wrote; I've got something that *mostly* works.

function _metaflac_tags () {
    local foo bar
    foo=$(metaflac $line[1] --export-tags-to=-) #Gets a list of the 
tags (and their values)
    bar=("${(@)${(f)foo}%[=]*}") #Cuts the list into an array just of 
the tag names (what I want)
    compadd -a bar -X "Existing tags"
}

This is called from:

_arguments '--show-tag=:FLAC tags:_metaflac_tags'

My questions are, first, is this the right way to do this?  Should I 
use some utility function instead? (maybe _alternatives or something?)

Second, assuming this *is* the right way to do this, how do I get a 
neat looking description line, like I get for all the other argument 
completions?  I assume I'm pre-empting it with my call to compadd, 
somehow, but I don't understand how this is happening.

Suggestions greatly appreciated!

Jesse Weinstein