Created attachment 416486 [details] tail of build log Having grass 6.4.2 installed, emerging grass 7.0.1-r3 resulted in quite a number of file collisions. On the other hand, unmerging grass using "emerge -c" would fail due to some unsatisfied dependency. I can't see why you had to introduce a new slot in the first place. If there is a good reason for doing so, then I think the two slots should block one another, so that portage knows that emerging one means unmerging the other, and that collisions between them are to be expected. If portage is as clever as I think it might be.
grass-6.4.2 has been masked for removal for some time and will be removed as part of the work in bug 566314.
commit 7b7292c071b7e6f0c8be5952cf2a05691d072222 Author: Ian Delaney <idella4@gentoo.org> Date: Tue Dec 1 16:38:06 2015 +0800 sci-geosciences/grass: remove pmasked and broken vn. 6.4.2 Package-Manager: portage-2.2.24
