Any standard function to prompt for multiple comma-separated items?

[Peter S Galbraith]

Hello,

For MH-E, I'm looking for code already available in Emacs to do
completing-read for multiple comma-separated items.  e.g.

To: p.gal[TAB]
->
To: p.galbraith
->
To: p.galbraith, psg[TAB]
->
To: p.galbraith, psg@home

Does anyone now of such code already existing?

There's multi-prompt.el bundled with AUC-TeX that comes close (doesn't
allow whitespace after the field separator), but I need something I can
use in MH-E without external packages.

Thanks,
-- 
Peter S. Galbraith, MH-E developer  <p.galbraith@globetrotter.net>
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