Bag

Check if a bag contains an element

Deletes an element and returns the new bag It doesn't complain if the element doesn't exist

Elements of a Bag with their multiplicity

Inserts an element in a bag and returns the new bag

Multiplicity of an element in a bag

add an element n times to a bag This default implementation is not efficient, it should be overridden

Size returns the total number of elements. Notice that if an element appears 4 times, it adds 4 to the counter

Union of two bags