[plt-scheme] In other PLaneT news: new feature

The PLaneT client in SVN now recognizes a new field in info.ss files: 
`can-be-loaded-with', which allows a package to specify that it may be 
loaded simultaneously with older versions of itself.

 From PLaneT's doc.txt:

--------
The _'can-be-loaded-with field_

If present, the can-be-loaded-with field should be a quoted datum of
one of the following forms:
  
can-be-loaded-with = 'all
                   | 'none
                   | (list 'all-except VER-SPEC ...)
                   | (list 'only VER-SPEC ...)

where VER-SPEC is a quoted instance of the VER-SPEC form defined at the
beginning of this document. [I.e., the numbers after the package name in 
a (require (planet ...)) form.]

Depending on your package's behavior, it may or may not be okay for
multiple versions of the same package to be loaded at one time on the
entire system --- for instance, if your package relies on writing to a
particular file and assumes that nothing else writes to that same
file, then multiple versions of the same package being loaded
simultaneously may be a problem. This field allows you to specify
whether your package can be loaded simultaneously with older versions
of itself. If its value is 'all, then the package may be loaded with
any older version. If it is 'none, then it may not be loaded with
older versions at all. If it is (list 'all-except VER-SPEC ...) then
any package except those that match one of the given VER-SPEC forms
may be loaded with this package; if it is (list 'only VER-SPEC ...)
then only packages that match one of the given VER-SPEC forms may be
loaded with this package.

When checking to see if a package may be loaded, PLaneT compares it to
all other currently-loaded instances of the same package with any
version: for each comparison, it checks to see if the newer package's
can-be-loaded-with field allows the older package to be loaded. If all
such comparisons succeed then the new package may be loaded; otherwise
PLaneT signals an error.

The default for this field is 'none as a conservative protection
measure. For many packages it is safe to set this field to
'any.
----------

I encourage feedback on this new feature.

-jacob