code: public static CompactSketch union(final CompactSketch skA, final CompactSketch skB) {
      return union(skA, skB, Util.DEFAULT_NOMINAL_ENTRIES);
    }
docs: This implements a stateless, pair-wise union operation on ordered,
  CompactSketches that are either Heap-based or Direct.
  If both inputs are null a null is returned.
  If one is null the other is returned, which can be either Heap-based or Direct.
  This is equivalent to union(skA, skB, k) where k is the default of 4096.
  
  @param skA The first ordered, CompactSketch argument.
  @param skB The second ordered, CompactSketch argument
  @return the result as an ordered CompactSketch.