s = "hoge"
p s[4,1] #=> ""
s[4, 0] = "e"
p s #=> "hogee"
とか
a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
p a[10,1] #=> []
a[10,0] = 0
p a #=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
みたいなのが嫌い