code: public static <T1, T2> void addInPlace(TwoDimensionalCounter<T1, T2> target, TwoDimensionalCounter<T1, T2> arg, double scale) {
      for (T1 outer : arg.firstKeySet())
        for (T2 inner : arg.secondKeySet()) {
          target.incrementCount(outer, inner, scale * arg.getCount(outer, inner));
        }
    }
docs: For all keys (u,v) in arg, sets target[u,v] to be target[u,v] + scale *
  arg[u,v]
  
  @param <T1>
  @param <T2>