Bag<E>
Implements the Iterable<E> trait.
A bag is an unordered collection of values of type E.
The following operations are supported for bags:
+ : (Bag<E>, Bag<E>) -> Bag<E>
Concatenates two bags.
in, !in : (E, Bag<E>) -> Boolean
Given a value and a bag, returns true if the bag contains the value (or not).