docs: public static <E> Counter<E> linearCombination(Counter<E> c1, double w1, Counter<E> c2, double w2) {
      Counter<E> result = c1.getFactory().create();
      for (E o : c1.keySet()) {
        result.incrementCount(o, c1.getCount(o) * w1);
      }
      for (E o : c2.keySet()) {
        result.incrementCount(o, c2.getCount(o) * w2);
      }
      return result;
    }