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