code: public CompactSketch aNotB(final Sketch a, final Sketch b) {
      return aNotB(a, b, true, null);
    }
docs: Perform A-and-not-B set operation on the two given sketches and return the result as an
  ordered CompactSketch on the heap.
  @param a The incoming sketch for the first argument
  @param b The incoming sketch for the second argument
  @return an ordered CompactSketch on the heap