a = [1, 2, 3, [4, 5, 6]]
p a.size # => 4
p a << a # => [1, 2, 3, [4, 5, 6], [...]]
p a.size # => 5
a[0] # => 1
a[3] # => [4, 5, 6]
a[3][0] # => 4
a[3].size # => 3
a[-2] # => [4, 5, 6]
a[-1] # => [1, 2, 3, [4, 5, 6], [...]]
a[a.size-1] # => [1, 2, 3, [4, 5, 6], [...]]
a[-1][-1] # => [1, 2, 3, [4, 5, 6], [...]]
a[-1][-1][-1] # => [1, 2, 3, [4, 5, 6], [...]]