stage in CERCLA cases, we must determine the appropriate standards for determining when apportionment is available and, when it is, how to ascertain the proper division of damages among defendants. Again, we draw on the experience of our sister circuits. The circuits that have addressed these questions have looked to common law principles of tort in general, and the Restatement in particular, for guidance as to when and how to impose joint and several liability under § 9607(a). We agree that this approach is proper and adopt it here. We also follow Chem-Dyne and all of the courts of appeals that have addressed the question in holding that the resulting standard must be a uniform federal rule. See, e.g., Aviall Servs., Inc. v. Cooper Indus., Inc., 312 F.3d 677, 684 (5th Cir.2002) (<HOLDING>), reversed on other grounds by 543 U.S. 157,
holding that state law determines successor liability under cercla recognizing cercla successor liability holding that geographic divisibility may provide a basis for apportionment of cercla liability holding that apportionment of cercla liability is  a matter of federal common law holding that actual apportionment of damages is a question of fact