* guix/profiles.scm (&profile-collision-error): New error condition. (manifest-transitive-entries, manifest-entry-lookup, lower-manifest-entry) (check-for-collisions): New procedures. (profile-derivation): Add call to 'check-for-collisions'. * guix/ui.scm (call-with-error-handling): Handle '&profile-collision-error'. * tests/profiles.scm ("collision", "collision of propagated inputs") ("no collision"): New tests.
