I'd like to add, seeing as slip and turn doesn't seem to understand - one method of seperation - either vertical seperation or lateral seperation more than adequate means of safely dealing with aircraft. If you had to have both in place, I think that the air traffic management in the LTMA would collapse. There would be extensive delays, aircraft would have to carry more and more fuel because they would be holding for longer in the stacks, ATCOs would become more stressed, so would pilots, and your perhaps looking at more chance of a collision. Lateral and vertical seperation is not an option. It's one or the other. And that's perfectly fine. Now you could get really fussy and never have aircraft go over each other, because even with 4,000ft of seperation, if that pilot decides too, in certain aircraft, he can cover that descent more than quick enough to cause an airprox and possibly a collision. Such seperation is impractical and unnecessary aslong as pilots comply with ATC instructions when inside CAS. If they don't it backs down to TCAS and (unreliable at the speeds most commericial jets travel at) the "see and avoid" principal.
Regards,
G_F.