'''Problem:''' Sometimes you don't want to require a particular package;
you just need a particular interface, and maybe many packages provide it.

[['''package require''']] operates on packages not interfaces, so what's
a good way to deal with alternative implementations of an interface?
(for example, C-coded and Tcl-coded versions of the same commands,
or perhaps competing implementations competing on other grounds?)

Main goal is to get complicated logic out of the user application.
A single declartive command should be sufficient.  ''I need version x
of the foo interface''

'''Option 1''' - a wrapper knows about multiple candidates

Write a common wrapper package that knows what packages provide a
common interface.  Have it supply configuration command to control
choosing.  Then the user just requires the wrapper package.

Wrapper can also translate any difference in the interfaces -- DB front end?

'''Option 2''' - alternatives know about each other

Index scripts / provide scripts of alternatives might be
written to recgonize avilability of an alternative and
less desirable one could refrain from calling [['''package ifneeded''']].
This is really a type of variant management, as it will be most
effective if the two alternatives are variants of a single version
of a single package.

'''Option 3''' - Accrue knowledge of interface compatibility on-the-fly.
Not hard-coded into any one package.

'''3a:''': an extension that will accept registration of claims of
support for named, versioned interface, with mechanisms for selecting
between alternatives.

'''3b:''': new subcommands of [['''package''']] instead of a separate extension.