不能将类型 []rune 用作附加类型的符文

Question

package mainvar lettersLower = []rune("abcdefghijklmnopqrstuvwxyz")var lettersUpper = []rune("ABCDEFGHIJKLMNOPQRSTUVWXYZ")func main() {    x := append(lettersLower, lettersUpper)}为什么这不起作用？我怎样才能附加lettersLower和lettersUpper？prog.go:7: cannot use lettersUpper (type []rune) as type rune in appendhttps://play.golang.org/p/ovx_o2rKPC