Assuming that this is a collection of collections, returns a new collection that contains all the values of each nested collection in order.
For collections that do not allow duplicate values, like
favor the last of all duplicates.
For maps, the iterables are treated as map-like objects and each successively
updates the result.
Flattening nested arrays is equivalent to concatenating each of them to an empty array.
Returns a new collection of the same type containing all the values of itself and the values of any number of other iterable collections in order.