不能将类型 []rune 用作附加类型的符文

Question

package main

var 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 append

https://play.golang.org/p/ovx_o2rKPC