Right now, 'merging and unmerging' has more of 'must not' and 'not specified' than actual rules. This means that practically doing any more complex file move in package requires hacking it around using pkg_preinst()/postinst(). I think we should develop a clear set of rules on how to replace anything with anything in a future EAPI, allowing us to finally be able to sanely move things some time in the future.
