In <jwvfwb0zvu0.fsf-monnier+gnu.emacs.help@gnu....> Stefan Monnier <monnier@iro....> writes:

>> Okay, as of now it looks like one cannot add any other repos; the
>> doc says:
>>     ;; This code supports a single package repository, ELPA.

>Not sure what definition of "now" you're using, ...

Well, I wrote that under the apparently erroneous impression that
the version I downloaded by following the instructions given in
tromey.com/elpa/install.html was the latest/"official" version of
package.el.  FWIW:

    % head ~/.emacs.d/elpa/package.el
    ;;; package.el --- Simple package system for Emacs

    ;; Copyright (C) 2007, 2008, 2009 Tom Tromey <tromey@redh...>

    ;; Author: Tom Tromey <tromey@redh...>
    ;; Created: 10 Mar 2007
    ;; Version: 0.9
    ;; Keywords: tools

    ;; This file is not (yet) part of GNU Emacs.


(This thread inadvertently/amusingly/ironically illustrates the
situation that motivated the initial query in the first place:
using publicly available Emacs extensions can be a lot trickier
than one might think...)