code: @NonNegative
    public int frequency(@NonNull E e) {
      if (isNotInitialized()) {
        return 0;
      }
  
      int hash = spread(e.hashCode());
      int start = (hash & 3) << 2;
      int frequency = Integer.MAX_VALUE;
      for (int i = 0; i < 4; i++) {
        int index = indexOf(hash, i);
        int count = (int) ((table[index] >>> ((start + i) << 2)) & 0xfL);
        frequency = Math.min(frequency, count);
      }
      return frequency;
    }
docs: Returns the estimated number of occurrences of an element, up to the maximum (15).
  
  @param e the element to count occurrences of
  @return the estimated number of occurrences of the element; possibly zero but never negative