I was reading how-much-is-the-minimum-safe-distance-between-two-planes-in-flight and realized part of it confused me (or maybe I'm just tired.)
In a low- or no-visibility environment (i.e. under IFR rules AND conditions), who is responsible for separation of planes in flight?
§91.111 Operating near other aircraft and §91.13 Careless or reckless operation are both shown in that question/answer, but if a flight is under ATC guidance, are they (ATC) or the Pilot-In-Command (PIC) responsible?
I'm most interested in the U.S. (FAA Regs), but discussions of other regulatory bodies would be fine.