关于go里面cap的结果的疑问

``` import "fmt" func main() { var a []int printSlice("a", a) // append works on nil slices. a = append(a, 0) printSlice("a", a) // the slice grows as needed. a = append(a, 1) printSlice("a", a) // we can add more than one element at a time. a = append(a, 2, 3, 4) printSlice("a", a) } func printSlice(s string, x []int) { fmt.Printf("%s len=%d cap=%d %v\n", s, len(x), cap(x), x) } ``` 运行结果： a len=0 cap=0 [] a len=1 cap=2 [0] a len=2 cap=2 [0 1] a len=5 cap=8 [0 1 2 3 4] 求大神指导下里面的cap值都是怎么得来的，特别是第二个是2，第三个也是2