fix(20): remove elements 3 and 4 from old array of figure 4

[AnruKitakaze]

I found an issue in Figure 4: elements 3 and 4 (indexes 4 and 5 respectively) should not appear in the original array. These slots remain uninitialized (zero values in example below), and the new elements should only exists in the newely allocated underlying array.

Here's a minimal example that demonstrates this behaviour:

package main

import "fmt"

func main() {
	s := make([]int, 4, 6)
	s[1] = 1
	s[3] = 2
	sliceStats("Initial underlying array", s)

	// no reassignment, to see old unerlying array
	appendExample(s)
	sliceStats("Old underlying array after append", s)

	fmt.Printf("No 3 and 4 in old underlying array*\n\n")

	// just for illustration
	changeUnderlyingArray(s)
	sliceStats("Changed underlying array in main", s)
}

func appendExample(s []int) {
	s = append(s, 3, 4, 5)
	sliceStats("New underlying array after append", s)
}

func changeUnderlyingArray(s []int) {
	s = append(s, 42)
	sliceStats("Changed underlying array in func", s)
}

func sliceStats(text string, s []int) {
	fmt.Printf("%s:\n\tlen = %d, cap = %d\n", text, len(s), cap(s))
	fmt.Printf("\tTo len: %v\n", s)
	fmt.Printf("\tTo cap: %v\n", s[:cap(s)])
}

Output:

go run . 
Initial underlying array:
        len = 4, cap = 6
        To len: [0 1 0 2]
        To cap: [0 1 0 2 0 0]
New underlying array after append:
        len = 7, cap = 12
        To len: [0 1 0 2 3 4 5]
        To cap: [0 1 0 2 3 4 5 0 0 0 0 0]
Old underlying array after append:
        len = 4, cap = 6
        To len: [0 1 0 2]
        To cap: [0 1 0 2 0 0]
No 3 and 4 in old underlying array*

Changed underlying array in func:
        len = 5, cap = 6
        To len: [0 1 0 2 42]
        To cap: [0 1 0 2 42 0]
Changed underlying array in main:
        len = 4, cap = 6
        To len: [0 1 0 2]
        To cap: [0 1 0 2 42 0]