code: public static CompactSketch aNotB(final Sketch skA, final Sketch skB) {
      if ((skA == null) && (skB == null)) { return null; }
      final short seedHash = (skA == null) ? skB.getSeedHash() : skA.getSeedHash();
      final HeapAnotB anotb = new HeapAnotB(seedHash);
      return anotb.aNotB(skA, skB, true, null);
    }
docs: This implements a stateless, pair-wise <i>A AND NOT B</i> operation on Sketches
  that are either Heap-based or Direct.
  If both inputs are null a null is returned.
  
  @param skA The first Sketch argument.
  @param skB The second Sketch argument.
  @return the result as an ordered CompactSketch on the heap.