static <A> double collectionBytesPerEntry(Cl<A> l) {
  L<A> data = asList(l);
  O[] a = toObjectArray_nonConcurrent(data);
  long arraySize = unsafe_sizeOf(a);
  long payloadSize = guessObjectSize(a)-arraySize;
  
  ret doubleRatio(guessObjectSize(l)-payloadSize, l(a));
}