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.
For collections that do not allow duplicate values, like Set, concat will
favor the last of all duplicates.
For maps, the iterables are treated as map-like objects and each successively
updates the result.
For iterators, concat constructs a new iterator that will exhaust the values
from each iterable in order, starting with this iterator.
concat is the variadic cousin of flatten.
Assuming that this is a collection of collections, returns a new collection that contains all the values of each nested collection in order.