docs: static DoublesUnionImpl directInstance(final int maxK, final WritableMemory dstMem) {
      final DirectUpdateDoublesSketch sketch = DirectUpdateDoublesSketch.newInstance(maxK, dstMem);
      final DoublesUnionImpl union = new DoublesUnionImpl(maxK);
      union.maxK_ = maxK;
      union.gadget_ = sketch;
      return union;
    }