code: public double incBy(K o, double inc) {
          Counter c = countMap.get(o);
          if (c == null) {
              c = new Counter();
              countMap.put(o, c);
          }
          c.count += inc;
          return c.count;
      }
docs: Increases the count of object o by inc and returns the new count value
  
  @param o
  @param inc
  @return