1a[0] is a t020 and also not null.
2a[1] is a t020 and also a k and also not null.
3a[2] is a t020 and also null.
4a[3] is a t020 and also null.
5
6Assigning a[0] to u failed.
7Assigning a[1] to u worked ok.
8Assigning a[2] to u worked ok.
9Assigning a[3] to u worked ok.
10