Recap - Custom List

Write CustomList in custom_list.gd, a class that wraps a private Array _items and offers what a list needs through methods and virtual methods:

• _init(items = []) stores a copy of items, so later changes to the Array passed in do not reach the list.
• append(item) adds an item at the end. pop_back() removes and returns the last item, or returns null when the list is empty.
• get_at(index) returns the item, or null when the index is outside 0 to size() - 1. set_at(index, value) replaces the item and returns true, or returns false for an index outside the list.
• size(), has(item) and clear() work like the Array methods with the same names.
• concat(other) takes the place of the + operator: it returns a new CustomList with this list's items followed by the items of other, and changes neither list.
• _to_string() returns the items the way an Array prints them, such as [3, 1, 4].
• _iter_init(), _iter_next() and _iter_get() let a for loop go through the items.

The supplied code reads the starting numbers (one line, separated by spaces, possibly empty), creates the list, appends 99 to the Array it passed in and prints the list. Then it reads the number of commands and runs one per line: append N, pop, get I, set I N, has N, size, sum (a for loop over the list), clear and concat N N .... For this input:

3 1 4
8
append 1
get 2
get 9
set 0 5
has 4
sum
pop
concat 9 2

it prints:

[3, 1, 4]
[3, 1, 4, 1]
Item 2: 4
No item at 9
[5, 1, 4, 1]
Has 4: true
Sum: 11
Popped 1
[5, 1, 4] + [9, 2] = [5, 1, 4, 9, 2]

Your code goes in custom_list.gd. main.gd holds the supplied input/output code and cannot be edited.