code: public static Histogram subtract(Histogram x, Histogram y) {
          return new Histogram(Stream.concat(
                  x.stream(),
                  y.stream().map(rwc -> {
                      rwc.setCount(-rwc.getCount());
                      return rwc;
                  })));
      }
docs: Subtracts two histograms.
  
  @throws IllegalArgumentException If the result contains mixed signs.