我不明白http://golang.org/pkg/text/template/ 中的文檔段落- The name of a key of the data, which must be a map, preceded  by a period, such as    .Key  The result is the map element value indexed by the key.  Key invocations may be chained and combined with fields to any  depth:    .Field1.Key1.Field2.Key2  Although the key must be an alphanumeric identifier, unlike with  field names they do not need to start with an upper case letter.  Keys can also be evaluated on variables, including chaining:    $x.key1.key2這是我的測試代碼，但失敗了。代碼：http : //play.golang.org/p/lbLJ4yoL2T。var season = map[int]string{1: "spring", 2: "summer",                      3: "autumn", 4: "winter"}                                  func main() {                                                              const greeting = `Welcome, {{.Key}}`                               t := template.Must(template.New("greet").Parse(greeting))          err := t.Execute(os.Stdout, season)                                if err != nil {                                                            fmt.Println(err)                                           }                                                          }                                                                  輸出Welcome, template: greet:1:11: executing "greet" at <.Key>: can't evaluate field Key in type map[int]string