code: public void incrementCount(F first, S second, double inc) {
          Counter<S> counter = maps.get(first);
          if (counter == null) {
              counter = new Counter<S>();
              maps.put(first, counter);
          }
  
          counter.incrementCount(second, inc);
      }
docs: This method will increment counts for a given first/second pair
  
  @param first
  @param second
  @param inc