code: private int _getRefCnt(Key<Vec> vec) {
      Integer I = REFCNTS.get(vec);
      assert I == null || I > 0;   // No zero or negative counts
      return I == null ? 0 : I;
    }
docs: Internal ref cnts (not counting globals - which only ever keep things alive, and have a virtual +1 to refcnts
  always).