The length of a slice may be change

The length of a slice may be changed as long as it still fits within the limits of the underlying array; just assign it to a slice of itself. The *capacity* of a slice, accessible by the built-in function `cap`, reports the maximum length the slice may assume. Here is a function to append data to a slice. If the data exceeds the capacity, the slice is reallocated. The resulting slice is returned. The function uses the fact that `len` and `cap` are legal when applied to the `nil` slice, and return 0.