I can merge two lists as follow together:

```
List(1,2,3) ::: List(4,5,6)

```

and the result is:

```
res2: List[Int] = List(1, 2, 3, 4, 5, 6)

```

the operator `:::` is right associative, what does it mean? 

In math, right associative is:

```
5 + ( 5 - ( 2 * 3 ) )

```