var/list/l = list(1, 2, 3, 4)
l[] = 5
had the same affect:
var/list/l = list(1, 2, 3, 4)
l[++l.len] = 5;
That means, something like this would be possible:
var/list/l = list(1, 2, 3, 4)
var/list/l2 = list(5, 6, 7, 8)
l[] = l2
where the final list would be: list(1, 2, 3, 4, list(5, 6, 7, 8)).
This would be preferred over the behavior of the + operator and list.Add proc, since those concat the lists; when sometimes you literally want to put a list into another list.