Traversable : Addition
- Operation ++
- Goal: appends two traversables together, or appends all elements of an iterator to a traversable.
> List(1, 2, 3) ++ List(4, 5, 6)
res0: List[Int] = List(1, 2, 3, 4, 5, 6)
concatate two lists
concatate an empty list with a list
