code: func inc(storage map[string]int, key string, i interface{}) {
  	cv := storage[key]
  	switch v := i.(type) {
  	case bool:
  		if v {
  			cv++
  		}
  	case int:
  		cv += v
  	}
  	storage[key] = cv
  }
docs: // inc makes sure that even for unused features, we initialize them in the
  // feature map. Furthermore, this acts as a helper that accepts booleans
  // to increment by one, or integers to increment by that integer.