code: func add(storage map[string]map[string]int, parent, child string, value int) {
  	n, ok := storage[parent]
  	if !ok {
  		n = make(map[string]int)
  		storage[parent] = n
  	}
  	n[child] += value
  }
docs: // add sets a key in a nested map, initializing things if needed as we go.