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I often change eel after the freeze in order to fix bugs in nautilus.
But I see the point in this, and its not a bad idea. How about limiting
it a bit though, so that after API freeze such libraries aren't allowed
to do api-incompatible changes.
This means you can still add functions, and in the libwnck case we have
here you could have added the new function, and then made the old one
call the new one. (This would require a new name for the function of
course, but at least you could fix the bug.)