code: static <T> VarOptItemsSketch<T> newInstanceAsGadget(final int k) {
      final VarOptItemsSketch<T> sketch = new VarOptItemsSketch<>(k, DEFAULT_RESIZE_FACTOR);
      sketch.marks_ = new ArrayList<>(sketch.currItemsAlloc_);
      return sketch;
    }
docs: Construct a varopt sketch for use as a unioning gadget, meaning the array of marked elements
  is also initialized.
  
  @param k   Maximum size of sampling. Allocated size may be smaller until sketch fills.
  Unlike many sketches in this package, this value does <em>not</em> need to be a
  power of 2.
  @param <T> The type of object held in the sketch.
  @return A VarOptItemsSketch initialized with maximum size k and a valid array of marks.