'''Problem:''' [['''package require''']] is limited in what requirements you can express.

''I need Tcl of at least version 7.5''

Best current expression:

======
    if {[catch {package require Tcl 8}]} {
        package require Tcl 7.5
    }
======

...and that will need more branches to support Tcl 9.

''I need foo 1.2 or better, except for foo 1.3.4, which I know has a bug I can't work around,
but I know it got fixed in 1.3.5, and it's still working with the current release 2.3.1''

Probably the best solution currently is to temporarily remove any [[package ifneeded  foo 1.3.4]]
registration before calling [[package require]].  If the bug covers many releases, though,
that can be tedious.

So, how about an extension of the [['''package require''']] syntax to:

'''package require''' ''$package ?$range ...?''

A range would have the form: ''$minVersion''?-''$maxVersion''?

The interpretation is that version ''$minVersion'' is acceptable,
and all versions greater than ''$minVersion'' and '''less than''' ''$maxVersion''
are acceptable.

As a special case, if $minVersion and $maxVersion are the same, then
only exactly that version is acceptable.  That is,

    package require bar 1.3.2-1.3.2

is equivalent to

    package require -exact bar 1.3.2

(but gets rid of the nasty switch!)

For compatibility, if only a single ''$minVersion'' is supplied, then
the default ''$maxVersion'' value is the next higher major version.
For example, 

    package require foo 1.2

is equivalent to

    package require foo 1.2-2

If the '''-''' appears, but no $maxVersion, then there is no upper limit.

For the examples above:

    package require Tcl 7.5-9

and when Tcl 9 is released, and we see that our program still works:

    package require Tcl 7.5-10

Or, if we want to make the users our testers for compatibility with
future Tcl:

    package require Tcl 7.5-

The second example:

    package require foo 1.2-1.3.4 1.3.5-3

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