What is a supported property?

All this resourcetype and state thing aside:

What is a supported property?

A resource has properties, let's call these existing properties,
which might or might not have values. But when a client does
a PROPFIND on them, he will get them listed in a propstat
element with 200 OK status code. I think that is a good definition
of an "existing property of a resource".

Now, every existing property would also be a supported property
and, being live, would appear in the supported-live-property-set.
Ok.

Now Geoff mentioned that a VCR with in-place editing would have
both DAV:checked-in and DAV:checked-out as supported properties,
and that independant of the checked in/out state of the resource!

Now, here I became confused, since it means that not every supported
property is an existing property! 

If we define supported properties with: 
	a property which will exist, when a method is applied successfully
then all non-versioned resources will have the DAV:checked-in as
supported property, since you can apply VERSION-CONTROL. So, that
does not seem to be a good definition...

And what about supported methods? Is CHECKIN a supported method for
a checked-in resource, too? It will fail all the time...

Geoff and Tim to the rescue, please!

Stefan